Systems and methods for advanced iterative decoding and channel estimation of concatenated coding systems

ABSTRACT

Systems and methods for decoding block and concatenated codes are provided. These include advanced iterative decoding techniques based on belief propagation algorithms, with particular advantages when applied to codes having higher density parity check matrices. Improvements are also provided for performing channel state information estimation including the use of optimum filter lengths based on channel selectivity and adaptive decision-directed channel estimation. These improvements enhance the performance of various communication systems and consumer electronics. Particular improvements are also provided for decoding HD Radio signals, including enhanced decoding of reference subcarriers based on soft-diversity combining, joint enhanced channel state information estimation, as well as iterative soft-input soft-output and list decoding of convolutional codes and Reed-Solomon codes. These and other improvements enhance the decoding of different logical channels in HD Radio systems.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims the benefit and is a continuation of U.S.patent application Ser. No. 13/693,023, filed Dec. 3, 2012. The presentapplication is related to co-pending, commonly assigned U.S. patentapplication Ser. No. 14/825,531 and U.S. patent application Ser. No.14/825,557 concurrently filed with the present application. The contentsof the above-identified applications are incorporated by reference intheir entirety as if recited in full herein.

FIELD OF THE INVENTION

The invention is directed to systems and methods for advanced iterativedecoding of multiple concatenated codes and advanced iterative channelstate information estimation for communication systems, and particularlyreceivers, used for HD Radio communication and other systems. Thesesystems and methods improve the performance of such receivers, therebyextending the range/coverage of digital radio preferably withoutmodifying the existing HD Radio infrastructure, and also minimizingpower consumption. While certain portions of the specification may focuson HD Radio technology, it should be understood that various aspects ofthe invention are also advantageous for, and may be used in, many otherapplications, standards and systems, including wireless or wiredbroadcasting and transmission systems, consumer electronics, storagemedia, computer applications and the like, as discussed in more detailin relevant portions of the detailed description.

BACKGROUND

Wireless or wired communications systems often rely onforward-error-correction (FEC) in order to control errors wheninformation is transmitted over noisy communications channels. In suchsystems, the sender encodes the information to be transmitted usingerror correcting codes. Exemplary error correcting codes include blockcodes (i.e., ones that operate on fixed-size packets), convolutionalcodes (i.e., ones that may operate on streams of arbitrary length), orconcatenated codes (i.e., ones that combine block codes andconvolutional codes). Certain block codes can be represented by paritycheck matrices, such as high, medium and low density parity check(H/M/LDPC) codes. Reed-Solomon (RS) codes are an example of well-knownblock codes as they are not only employed in many wireless or wiredcommunication systems such as broadcast systems (including HD Radiosystems which are discussed further below), but also in consumerelectronics and data storage systems such as disc drives, CDs and DVDs.

While many methods exist for decoding of LDPC codes such as ones basedon belief propagation (BP) algorithms, such methods typically do notyield good performance when used to decode codes having higher paritycheck matrix densities, including MDPC and HDPC codes, such as RS or BCHcodes. Thus, there is a need for improved systems and methods fordecoding block codes (or concatenated codes that include block codes),particularly H/M/LDPC codes or any codes that could be represented byparity check matrices including RS codes, in a manner that improves theperformance while keeping computational complexity reasonable.

Proper FEC decoding in communications systems also relies on the abilityto determine as best as practically possible certain attributes of thecommunication channel. For instance, the channel response as well asnoise power estimation, which together may be referred to as channelstate information (CSI), are often estimated and used not only for errorcorrection, but also for other processing functions such as coherentdemodulation and diversity combining in order to achievemaximum-possible performance gains offered by those processing tasks.CSI estimation is also of importance in diversity combining systems.

To facilitate CSI estimation, pilot symbols are usually inserted in astream of data symbols. Such pilot channel estimation relies onfiltering techniques that have typically used filter lengths whicheither do not optimally account for noise effects or channel dynamics(i.e., the rapidity of channel variation). Thus, although prior artfilter structures may be suitable for certain scenarios, they are notoptimal when the system needs to operate over a wide range of channeldynamics, thereby adversely affecting performance. Accordingly, there isalso a need to improve channel estimation techniques in additive whiteGaussian noise (AWGN) and fading communication channels, which wouldresult in improved decoding performance.

As discussed above, there is a need for improved techniques for decodinga wide variety of codes, including RS codes, which may be used invarious systems including consumer electronics and data storage systems,as well as broadcast systems (where there is also a need to improvechannel estimation) such as in HD Radio receivers. HD Radio refers to adigital radio technology that enables the transmission and reception ofdigital audio and data, addressing the limitations of aging analogbroadcast transmission technology.

Current HD Radio systems are based on a particular type of multicarriertechnology known as orthogonal frequency-division multiplexing (OFDM). Ahybrid method of transmitting analog radio broadcast signals and digitalradio signals simultaneously on the same frequency band is referred toas in-band on-channel (IBOC) transmission. IBOC transmission allowsbroadcasters to transmit both analog and digital signals on theirexisting assigned channels in the amplitude modulation (AM) or frequencymodulation (FM) frequency range. On the other hand, all-digital HD Radiosystems of the future (which are not yet deployed) are expected to onlycarry the digital HD Radio signal.

HD Radio systems typically transmit a system control data sequence forthe purpose of system control data synchronization and, possibly,channel estimation. For example, the system control data sequence in FMHD Radio consists of synchronization bits, control bits, and paritybits, which are transmitted on pilot tones that are commonly referred toas the reference subcarriers. The differential phase-shift keying (DPSK)modulated pilot symbols are multiplexed onto OFDM symbol along with datasymbols. The reference subcarriers on which pilot symbols aretransmitted are distributed over the OFDM spectrum. Control and statusinformation are collected to form system control data sequences and aretransmitted on the reference subcarriers. Use of the system control datasequence for acquisition, tracking, channel estimation and coherentdemodulation has been described in U.S. Pat. No. 6,549,544. Decoding ofthe system control data sequence is important for the systemperformance. The parity bits are inserted into the variable fields ofthe system control data sequence for error detection and prevention oferror propagation at the end of each variable field due to differentialencoding.

The DPSK modulated pilot symbols, in which the information is carried inthe phase difference between adjacent bits, are decoded non-coherentlyat the receiver. Selected information bits in a system control datasequence may be repeated within the same system control data sequenceand those repeated bits are transmitted on a set of referencesubcarriers whose positions in the frequency band are known to thereceiver such that frequency diversity could be exploited during thedecoding process at the receiver.

In the current HD Radio receivers, all transmitted DPSK-modulated systemcontrol data sequences carried on the reference subcarriers are firstnon-coherently demodulated and then a majority voting is applied tothose repeated bits to make a final bit decision of all repeated bitscollectively. The final bit decision based on majority votingfacilitates a correct decoding of those bits repeated in the systemcontrol data sequence, although some of the repeated bits in a systemcontrol data sequence may be corrupted when received. This process iscommonly referred to as majority voting combining. In addition to therepetition of some bits in a system control data sequence, a small setof bits in a system control data sequence are protected by a parity bit,allowing detection of existence of bit errors in the set ofparity-covered bits.

As for channel estimation, if the parity does not match, the parityfield is considered unreliable and may not be used to estimate thechannel response (or noise power). In this case, non-uniforminterpolation could be applied.

In addition, existing HD Radio receivers rely on Viterbi decoders todecode convolutional codes in a manner that produces hard-decisiondecoded bits. For audio channels, these hard-decision outputs are passedto a conventional cyclic redundancy check (CRC) decoder for errordetection, and then to a source audio decoder. For data channels,hard-decision outputs are passed to an algebraic RS decoder, alsoproducing hard decision bits, followed by a conventional CRC decoder forerror detection. Each operation is done once and in a sequential mannerin the prior art. However, algebraic RS decoding on hard bit decisionsout of the Viterbi decoder results in suboptimum performance and such anapproach is not amenable to potential iterative decoding improvements.

From the foregoing discussion, it is clear that there is a need forimproving the performance of decoders, and more generally, decodingblock codes (or concatenated codes that include block codes), as well aschannel estimation, in communication and other systems. Moreover, thereparticularly is a need for systems and methods that improve the decodingperformance of AM and FM HD Radio receivers in order to extend therange/coverage of digital radio, preferably without modifying theexisting HD Radio transmission or infrastructure.

SUMMARY

Systems and methods are provided for improving the performance of FERdecoders, including decoding of any block codes that can be representedby parity check matrices (or concatenated codes that include such blockcodes) through advanced iterative decoding techniques based on BPalgorithms, with particular advantages when applied to RS codes andother codes characterized with higher density parity check matrices.

According to these systems and methods where a code may be representedby a parity check matrix of dimension (N−K)×N, one or more parity checkmatrices with N−K sparse columns may be generated. Up to N−K sparsecolumns may contain only a single entry equal to 1 per column, whereinsparse columns of each of P parity check matrices correspond todifferent subsets of N−K bit log-likelihood ratios of N−K+R leastreliable bit log-likelihood ratios, where R≧P is a configurable integerdefining R least reliable bits in the dense part of the parity checkmatrix if only a single parity check matrix were used. The channellog-likelihood ratios (LLRs) may be decoded using the P parity checkmatrices with sparse columns to produce updated LLRs. This may be doneusing soft-input soft-output (SISO) message passing decoding until adesired number of iterations is reached or until the decoding using atleast one of the P matrices produces a valid codeword. If no validcodeword is produced, additional decoding that is based at least in parton algebraic decoding of the sequences of the updated LLRs may beperformed.

The SISO message passing decoding algorithm may be based on beliefpropagation algorithms or variations thereof. The SISO message passingdecoding may generate check-variable messages and scale them by a factor1−β·Min₁/Min₂, where 0≦β≦1, and Min₁ and Min₂ may be the two smallestabsolute values in a set of variable-to-check messages. Alternatively,or in addition, SISO message passing decoding may include simple greedyscheduling of check equation updates, where metric values Val_(i), i=1,2, . . . , L and L≧1 for scheduling order determination may becalculated for a set of non-updated check nodes. Val_(i) may becalculated as Val_(i)=Min₁+Min₂, and the set {Val_(i)} may be sorted indecreasing order to obtain an ordering vector, such that the L checknode equations may be updated according to this vector, andcorresponding check-to-variable messages may be calculated andpropagated. Variable-to-check messages may also be updated for allvariables that received check-to-variable messages and these steps maybe repeated until all check nodes are updated by calculating andpropagating corresponding check-to-variable messages. Message passingupdates may be calculated using a min-sum, sum-product, or any otherappropriate algorithm. During the course of SISO message passingdecoding iterations, one or more of parity check matrices may be updatedbased on updated bit LLRs.

In certain situations, variable-to-check messages may have differentsigns in two consecutive iterations, suggesting that convergence forthat node is not achieved yet. In some embodiments, variable-to-checkmessages that have not converged are modified such that the outgoingvariable-to-check message is equal to a weighted average of such twonon-converged consecutive messages with different signs. The weightingfactor may depend upon the specific code and, typically, more weight isgiven to the more recent message.

Systems and methods are also provided for performing advanced CSIestimation which enhances pilot channel estimation by using an optimumfilter length, adaptive decision-directed channel estimation and/orperforming the estimation iteratively using shorter filters. Forexample, filter lengths may be selected based on channel selectivitywhich is estimated using known pilot symbols and a fraction of unknowndata symbols. With these selected filter lengths, estimation of thechannel response may be performed using available pilot symbols and datasymbols that are considered relatively reliable. As for data symbolsthat may be considered less reliable, the channel response may beestimated by interpolation using estimated values of channel responseobtained from the pilot symbols and the more reliable data symbols. Thechannel response estimates can be used to estimate noise power and/orimprove FEC decoding, and multiple estimation and FEC decodingiterations can be performed. In subsequent iterations, the length of oneor more filters may be decreased and the fraction of data symbolsobtained using the improved FEC decoding may be increased.

An exemplary criterion for selecting a fraction of more reliable datasymbols may be based on the channel response at a given symbol positionbeing larger than a first threshold, or the signal-to-noise ratio (orthe signal-to-noise-plus-interference ratio) at a given symbol positionbeing larger than a second threshold, where the thresholds may bedetermined based on the estimated channel selectivity.

Moreover, systems and methods are particularly provided for decodingdigital radio signals, including hybrid digital radio signals, inmulticarrier systems such as ones that encode digital data using OFDM.These systems and methods improve HD Radio receivers by, for example,relying on soft-diversity combining of soft values of repeated controldata bits carried on a plurality of reference subcarriers, techniquesfor collectively utilizing soft combining and multiple symbol detection,and/or techniques for correcting bit errors using parity check bits.They also utilize the corrected parity check bits as part ofsoft-diversity combining and/or as part of the collective use ofsoft-diversity combining and multiple symbol detection.

For example, a hybrid digital radio OFDM signal may be decoded byperforming initial CSI estimation using distorted modulated symbolsobtained from the received OFDM signal, producing soft estimates ofconvolutional code coded bits using SISO decoding of one or moreconvolutional codes associated with one or more logical channels carriedin a radio frame of the OFDM signal, performing at least one additionaliteration of CSI estimation by using at least some of the softconvolutional code coded bits estimates, and decoding any logicalchannel using improved CSI obtained by the at least one additional CSIiteration. Exemplary logical channel information whose decoding may beimproved include program service data (PSD) protocol data units (PDUs),Primary IBOC Data Service (PIDS) PDUs, station information service (SIS)PDUs, advanced application service (AAS) PDUs, main program service(MPS) PDU headers, and/or audio packet data.

CSI estimation for such an OFDM multicarrier system may be performedusing the techniques stated above. For example, part of this processinvolves estimating unknown symbols on the reference subcarriers, whichmay be done by soft diversity combining of control data sequence symbolsthat carry the same symbol value, and differentially decoding the softdiversity combined symbols to obtain corresponding decoded control datasequence bits. Thereafter, the control data sequence symbols may bereconstructed from the decoded control data sequence bits. In certainembodiments, the control data sequence symbols may be reconstructed froman improved decoded control data sequence using single parity check codebits where the least reliable soft decoded control data sequence bit isflipped if the parity does not check (otherwise, no bit is flipped).Moreover, SISO list Log-MAP decoding of the employed convolutional codesmay be employed in conjunction with the CSI estimation herein.

The above systems and methods improve HD Radio receiver performance notonly by helping to decode control data sequences more reliably, but alsoby facilitating better CSI estimation that employs referencesubcarriers, which contributes to improved decoding of all HD Radiological channels.

SISO decoding of convolutional codes, instead of using a Viterbialgorithm, is used to facilitate iterative CSI estimation but also softdecoding of the outer RS codes for applicable logical channels. In someembodiments, in addition to soft outputs, a list of most likelysequences is produced, which is referred to as list SISO decoding.Improved CSI estimation is especially beneficial in combination withlist SISO decoding by enabling better estimation of a candidate list ofmost likely sequences.

Moreover, a SISO RS decoder may be used to decode soft information bitsoutputs of the convolutional code. List SISO decoding is alsoadvantageous when used in combination with decoding of outer RS codesfor applicable logical channels as it could significantly reduce thedecoding complexity. This is so given that outer SISO RS decoding wouldbe required only in a relatively small fraction of cases, as list SISOconvolutional decoding would produce a correct sequence on the list mostof the time. These advanced RS decoding techniques also lead toimprovements in HD Radio system performance by enabling better decodingof data channels as well as headers present in audio channels (e.g., MPSPDUs), thereby further improving HD Radio receivers. The foregoingimprovements result in extending the range/coverage of digital radioreceivers, without modifying the existing HD Radio system infrastructureor the air-interface.

Other benefits and features of the present invention, including itsapplicability to other areas of wireless transmission systems, as wellas wired transmission systems, consumer electronics, storage media, andcomputer applications may become apparent from the following detaileddescription considered in conjunction with the accompanying drawings. Itis to be understood, however, that the drawings are designed solely forpurposes of illustration and not as a definition of the limits of theinvention, for which reference should be made to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which incorporate in and constitute a part ofthe specification, illustrate preferred embodiments of the invention andtheir relationship to the prior art, and together with the detaileddescriptions serve to explain the principles and advantages of variousaspects of the invention:

FIG. 1 is a simplified block diagram of a transmitter and receiver,encompassing a single FEC encoder/decoder, modulator/demodulator andchannel;

FIG. 2 is a simplified block diagram of a transmitter and receiver withconcatenated FEC, encompassing outer and inner FEC encoders/decoders,modulator/demodulator and other relevant transmitter and receiverblocks;

FIG. 3 is a simplified factor graph of a linear code defined by a paritycheck matrix illustrating flooding BP and a short cycle;

FIG. 4 is a simplified factor graph of a linear code defined by a paritycheck matrix illustrating informed BP scheduling using Residual BP (RBP)algorithm;

FIG. 5 is a simplified factor graph of a linear code defined by a paritycheck matrix illustrating simple greedy BP decoding in accordance withcertain embodiments of the invention;

FIG. 6 is a flow diagram illustrating embodiments for decoding,particularly showing relevant elements and process steps in accordancewith certain embodiments of the invention;

FIG. 7 is a chart illustrating the performance of RS (255,239), such asusing multiple parity check matrices, simple greedy BP and simple greedyBP combined with Berlekamp-Massey (BM) decoding in accordance withcertain embodiments of the invention;

FIG. 8 is a chart illustrating the performance of RS (255,239) using thecertain embodiments of the invention compared to known methods in theprior art in accordance with certain embodiments of the invention;

FIGS. 9a, 9b, 9c, 9d, and 9e show typical arrangements of pilot symbolsin single carrier systems, where FIG. 9a illustrates a continuous streamof pilot symbols (also referred to as the pilot channel), FIG. 9billustrates a preamble in a stream of data and pilot symbols, FIG. 9cillustrates a midamble in a stream of data and pilot symbols, FIG. 9dillustrates interleaved pilot symbols, and FIG. 9e illustrates multiplepilot symbol bursts in a stream of data and pilot symbols;

FIGS. 10a and 10b illustrate typical arrangements of pilot symbols inmulticarrier systems, where FIG. 10a illustrates an arrangement ofdedicated pilot subcarriers, and FIG. 10b illustrates an arrangement ofinterleaved and staggered pilot symbols;

FIGS. 11a, 11b, and 11c illustrate channel response dynamics in fadingchannels, where FIG. 11a illustrates the response dynamics in a slowfading channel, FIG. 11b illustrates the response dynamics in amoderately fast (or medium-speed) fading channel, and FIG. 11cillustrates the response dynamics in a fast fading channel;

FIG. 12 illustrates a conceptual realization of how to determine optimumfilter lengths in certain embodiments, taking into account noise effecton channel estimation but also mismatch between filter lengths andchannel dynamics;

FIGS. 13a and 13b show illustrative examples of noise plus interferenceprofiles, where FIG. 13a illustrates the profile over time and FIG. 13billustrates the profile over frequency in multicarrier systems;

FIGS. 14a, 14b, 14c, and 14d illustrate impacts of signal-to-noise ratio(SNR) threshold on the mixture of pilot and decision-directed channelestimation in accordance with certain embodiments, where FIG. 14aillustrates the channel response over time in slow fading channels(i.e., longer filter lengths), FIG. 14b illustrates the channel responseover time in fast fading channels (i.e., shorter filter lengths), FIG.14c illustrates the channel response over frequency with a higherthreshold, and FIG. 14d illustrates the channel response over frequencywith a lower threshold;

FIG. 15 shows a flow chart of the initial algorithm for CSI estimation;

FIG. 16 shows an illustrative block diagram for joint, iterative CSIestimation and SISO decoding;

FIG. 17 illustrates time-varying signal-to-noise-plus-interferenceratios (SNIRs) for an example of two diversity channels in accordancewith certain embodiments of the invention;

FIG. 18 is a block diagram of a transmitter and receiver withconcatenated FEC, encompassing outer and inner FEC encoders/decoders,outer and inner interleavers/de-interleavers, symbol mapper/de-mapper,channel, advanced CSI estimation as well as the possible iterativestructure between inner and outer FEC decoders and the possibleiterative structure between the advanced CSI estimation and the innerFEC decoder in accordance with certain embodiments of the invention;

FIG. 19 is a block diagram of encoding of a general multilevel code withM levels, encompassing partitioning of information, individual FECencoders and interleavers for each level as well as symbol mapper;

FIG. 20 is a block diagram of decoding of a general multilevel code withM levels using multistage decoding, encompassing individual de-mappers,FEC decoders and de-interleavers for each level and transmission ofdecoded information through interleavers from lower level FEC decodersto the higher level de-mappers as well as the possible iterativestructure from higher level FEC decoders to the lower level de-mappers;

FIG. 21 shows a 16-QAM (Quadrature Amplitude Modulation) hierarchicalmodulation scheme where both the base and enhancement layers are encodedusing Quadrature Phase Shift Keying (QPSK) modulation such that the baselayer bits have larger Euclidean distance than the enhancement layerbits;

FIG. 22 shows an 8-PSK (Phase Shift Keying) hierarchical modulationscheme where the two base layer bits are QPSK encoded and theenhancement layer is one bit and further showing larger Euclideandistance for base layer bits than for the enhancement layer bits;

FIG. 23 is a block diagram of a transmitter and receiver withconcatenated FEC and hierarchical modulation encompassing outer andinner FEC encoders/decoders and outer interleaver/de-interleaver forboth base and enhancement layers, inner interleaver/de-interleaver,hierarchical symbol mapper/de-mapper, channel, advanced CSI estimationas well as the possible iterative structure between inner and outer FECdecoders and the possible iterative structure between the advanced CSIestimation and the inner FEC decoder for both base and enhancementlayers in accordance with certain embodiments of the invention;

FIG. 24a is a simplified block diagram illustrating the functionality ofan HD Radio transmitter that multiplexes a plurality of informationsequences into a single stream in the time domain;

FIG. 24b is a simplified block diagram illustrating the functionality ofan HD Radio receiver in accordance with certain embodiments of theinvention;

FIG. 25 is a simplified block diagram illustrating the functionality ofan HD Radio transmitter that multiplexes a plurality of binary sequencesinto a single stream in time domain;

FIG. 26 is a simplified block diagram illustrating the functionality ofan HD Radio receiver in accordance with certain embodiments of theinvention;

FIG. 27 is a block diagram of an FM HD Radio OFDM system showingmultiplexing/demultiplexing of reference and data subcarriers;

FIG. 28 depicts control data sequence fields in FM HD Radio systems;

FIG. 29 is a conceptual representation of reference subcarriers(carrying the system control data sequence) and data subcarriers intime-frequency plane in FM HD Radio systems;

FIG. 30 is a diagram of processing flow for decoding of the systemcontrol data sequence bits with hard decisions and majority voting inconventional receivers;

FIG. 31 is a diagram of processing flow for decoding of the systemcontrol data sequence bits with soft diversity combining in certainembodiments of the invention;

FIG. 32 is a diagram of processing flow for decoding of the systemcontrol data sequence bits with soft diversity and multiple DPSK symbolinterval detection;

FIG. 33 is a diagram of processing flow for decoding of the systemcontrol data sequence bits with parity bit correction;

FIG. 34 illustrates the computer-simulated bit error rates (BERs) of thecontrol data sequence of the FM HD Radio receivers in the prior art ascompared to certain embodiments in a URBAN-SLOW (abbreviated as USLOW)multipath fading channel with a mobile speed of 2 kilometer per hour[KPH];

FIG. 35 illustrates the computer-simulated frame error rates (FERs) ofthe control data sequence of the FM HD Radio receivers in the prior artas compared to certain embodiment in a USLOW multipath fading channelwith a mobile speed of 2 KPH;

FIG. 36 illustrates the computer-simulated BERs of the control datasequence of the FM HD Radio receivers in the prior art as compared tocertain embodiments in a URBAN-FAST (abbreviated as UFAST) multipathfading channel;

FIG. 37 illustrates the computer-simulated FERs of the control datasequence of the FM HD Radio receivers in the prior art as compared tocertain embodiments in a UFAST multipath fading channel;

FIG. 38 illustrates the signal structure including frequency partitionsof OFDM signal in the frequency domain of hybrid FM HD Radio systems;

FIGS. 39a, 39b, and 39c illustrate simplified structures of PDUs in theHD Radio system, where FIG. 39a is a simplified structure of the MPS PDUillustrating a PDU header, a plurality of audio packets, and PSD onlogical channel P1, FIG. 39b is a simplified structure of the SIS PDU onlogical channel PIDS, and FIG. 39c is a simplified structure of the AASPDU on logical channel P3;

FIG. 40 is a diagram of processing flow for decoding of the MPS PDUheader in accordance with certain embodiments of the invention;

FIG. 41 is a diagram of processing flow for decoding of the PSD inaccordance with certain embodiments of the invention;

FIG. 42 is a diagram of processing flow for decoding of the audio packetin accordance with certain embodiments of the invention;

FIG. 43 is a diagram of processing flow for decoding of the SIS PDU inaccordance with certain embodiments of the invention;

FIG. 44 is a diagram of processing flow for decoding of the AAS PDU inaccordance with certain embodiments of the invention;

FIG. 45 is a diagram of processing flow for byte de-interleaving of LLRsand list sequences at the output of list Log-MAP decoder in accordancewith certain embodiments of the invention;

FIG. 46 is a diagram of iterative decoding of MPS, SIS, and AAS PDUs inaccordance with certain embodiments of the invention;

FIG. 47 illustrates the computer-simulated FERs of audio packets of theFM HD Radio receivers in the prior art and in certain embodiments of theinvention in a USLOW multipath fading channel with a mobile speed of 2KPH and in the presence of host FM interference;

FIG. 48 illustrates the computer-simulated FERs of audio packets of theFM HD Radio receivers in the prior art with CSI estimated from referencesubcarriers in a USLOW multipath fading channel with a mobile speed of 2KPH and in the presence of host FM interference;

FIG. 49 illustrates the computer-simulated FERs of audio packets of theFM HD Radio receivers in the prior art and in certain embodiments of theinvention in a USLOW multipath fading channel with a mobile speed of 2KPH, and in the presence of host FM plus 1^(st) adjacent FMinterference;

FIG. 50 illustrates the computer-simulated FERs of audio packets of theFM HD Radio receivers in the prior art and in certain embodiments of theinvention in a UFAST multipath fading channel with a mobile speed of 60KPH, and in the presence of host FM plus 1^(st) adjacent FMinterference;

FIG. 51 illustrates the computer-simulated FERs of audio packets of theFM HD Radio receivers in the prior art and in certain embodiments of theinvention in a 3-ray (abbreviated as 3RAYS) multipath fading channel at100 KPH, and in the presence of host FM plus 1^(st) adjacent FMinterference;

FIG. 52 illustrates the computer-simulated FERs of audio packets of theFM HD Radio receivers in the prior art and in certain embodiments of theinvention in a UFAST multipath fading channel with a mobile speed of 60KPH and in the presence of host FM plus 1^(st) adjacent FM interference;the digital signal level is boosted by 6 dB, as opposed to 10 dB in FIG.50 and other figures, relative to the level allowed by the original HDRadio standard, according to an FCC Order in 2010;

FIG. 53 illustrates the computer-simulated FERs of program PSD PDUs ofthe FM HD Radio receivers in the prior art and in certain embodiments ofthe invention in a USLOW multipath fading channel with a mobile speed of2 KPH, and in the presence of host FM plus 1^(st) adjacent FMinterference;

FIG. 54 illustrates the computer-simulated FERs of main program serviceMPS PDU headers of the FM HD Radio receivers in the prior art and incertain embodiments of the invention in a USLOW multipath fading channelwith a mobile speed of 2 KPH, and in the presence of host FMinterference;

FIG. 55 illustrates the computer-simulated FERs of main program serviceMPS PDU headers of the FM HD Radio receivers in the prior art and incertain embodiments of the invention in a USLOW multipath fading channelwith a mobile speed of 2 KPH, and in the presence of host FM plus 1^(st)adjacent FM interference;

FIG. 56 illustrates the computer-simulated FERs of PIDS frames of the FMHD Radio receivers in the prior art and in certain embodiments of theinvention in a 3RAYS multipath fading channel, and in the presence ofhost FM plus 1^(st) adjacent FM interference; and

FIG. 57 illustrates the computer-simulated FERs of P3 AAS data of the FMHD Radio receivers in the prior art and in certain embodiments of theinvention in a UFAST multipath fading channel with a mobile speed of 60KPH, and in the presence of host FM plus 1^(st) adjacent FMinterference.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Certain embodiments are directed to systems and methods for advancediterative decoding of multiple concatenated codes and advanced iterativechannel state information estimation for communication systems. Whilemany of the embodiments in this section are discussed in the context oftelecommunications systems, with some focus on HD Radio receivers, itshould be understood that various aspects of the invention are alsoadvantageous for, and may be used in, many other applications, standardsand systems, including many wireless or wired broadcasting andtransmission systems, consumer electronics, storage media, computerapplications and the like, as discussed in more detail in variousportions of this section that follow.

This section is organized into four major subsections, each pertainingto different aspects which relate generally to improvements in decodingand channel estimation, and which are all applicable to HD Radiocommunication systems, and specifically to receivers/decoders in suchsystems which use existing standards, as well as other systems mentionedthroughout. The first subsection focuses on improved decoding ofmultiple concatenated codes, and discusses error correction techniquesthat use block codes, and in particular, certain parity check codes suchas high, medium and low density parity check codes. The secondsubsection focuses on improved CSI estimation techniques which can beused, in part, to enhance error correction and decoding. The thirdsubsection discusses the applicability of the decoding and CSIestimation techniques to various concatenating coding systems andstandards mentioned in the beginning of the third subsection, as well asadditional aspects, to improve the decoding in such systems. Finally,the fourth subsection discusses certain embodiments for iterativelydecoding HD Radio signals, including system control data sequences andlogical channel data in FM HD Radio.

I. Advanced Decoding of High/Medium/Low Density Parity Check Codes

This subsection discusses techniques for decoding of high, medium andlow density parity check (H/M/LDPC) codes, as well as any codes thatcould be represented by parity check matrices. Specifically, one aspectof the invention discussed herein pertains to iterative soft decodingtechniques based on belief propagation (BP) algorithms. The proposedsystems and methods could be used for decoding of standalone H/M/LDPCcodes or concatenated codes where one of the component codes is anH/M/LDPC code. These techniques provide improved performance withaffordable computational complexity, with particular advantagesillustrated when applied to Reed-Solomon (RS) codes. These aspects andcertain corresponding embodiments could be used in wireless and wirelinecommunications, as well as in any scenario where H/M/LDPC codes areemployed. For example, these aspects are applicable to HD Radio systems,as well as other wireless systems (mobile cellular, wireless LAN,microwave or satellite link, broadcast, mesh or ad-hoc network,infrared, underwater acoustic, etc.), wireline systems (coaxial cable,copper wires, fiber optic, etc.), or even storage media (disc, drive,magnetic tape, etc.).

In certain embodiments of the invention, an adaptive normalized min-sumalgorithm (MSA) is applied to improve the MSA performance. A noveladaptive normalized scaling factor is preferably tailored to MSA. Insome embodiments, weighted averaging of non-converged variable to checkmessages is employed. For variable to check node messages with differentsigns in consecutive iterations, a weighted average of messages might beadvantageous for some codes. In one embodiment, a simple greedy BPscheduling is employed in an attempt to perform decoding of checkequations in a particular, advantageous order, but without calculatingall the residual values in every iteration as in informed BP schedulingfor instance. It is based on the notion of first updating the checkequations with less chances of having errors in positions correspondingto the dense part of the parity check matrix. The simplicity of themethod comes from the fact that for each check node/equation, only onescheduling metric value is preferably calculated and assigned based ononly outgoing messages. Based on these values, an order for updatingcheck equations is obtained. In another embodiment, an alternativesimple greedy BP algorithm is used where, instead of ordering the metricvalues of all check equations at the beginning, only the check equationwith maximum value is preferably determined and updated. Then the metricvalues of all check equations are re-evaluated and, again, the nextcheck equation with maximum metric value is determined and updated. Thisprocess may be repeated until all the check nodes/equations arepreferably updated. In one embodiment, improved best graph BP decodingusing multiple parity check matrices is introduced. This method usesmultiple parity check matrices with different sparse and dense parts inorder to capture and correct more errors compared to a single matrix.This method employs simple greedy BP algorithm to perform BP iterationsfor each matrix. At the end of iterations, a final decoding, based atleast in part on algebraic decoding, is performed to the LLRs of notconverged matrices. In other embodiments, in improved best graph BPdecoding using multiple parity check matrices, instead of the simplegreedy BP algorithm, one can use any of the other BP decoding methodssuch as flooding BP (sum-product algorithm (SPA), MSA, normalized MSA,etc.) as well as informed BP scheduling (RBP, Node-Wise Scheduling(NWS), etc.). In another embodiment, the final decoder applied to theLLRs of not converged matrices can be improved using error and erasuredecoding. Considering the fact that the sign disagreement positionsbetween the updated LLR vectors of different matrices have a highprobability of being actually bit errors, in another embodiment,disagreement positions are forced into sparse parts of the matrices inorder to avoid the error propagation. In one embodiment, combination ofthe improved best graph BP decoding using multiple parity check matricesand the alternative with disagreement positions is employed. First theoriginal proposed decoder is applied. If it failed, the alternative withdisagreement positions is applied. In the final embodiment, the proposedmethods are used and discussed for decoding RS codes which arecharacterized with high density binary image parity check matrices.

Many existing methods in the art for decoding of high, medium and lowdensity parity check H/M/LDPC codes are based on BP. To explain thedecoding of codes represented by a parity check matrix, consider firstthe system model shown in FIG. 1. It shows a simplified block diagram ofa communication system including forward-error-correction (FEC) encoder101 and decoder 105, symbol mapper/de-mapper 102 and 104 andcommunication channel 103. As referenced above, the communicationchannel may be any wireless or wireline channel, or a storage medium, orany other example where H/M/LDPC codes are employed. In this figure,channel encoding using a (n,k) linear block code 101 is performed byconverting k information bits (symbols) 107 to n coded bits (symbols)108 followed by symbol mapper 102 to map coded bits (symbols) tomodulation symbols 109. The modulation symbols go through a propagationchannel 103 and at the receiver noisy modulation symbols 110 arereceived. Symbol de-mapping 104 converts noisy modulation symbols intocoded bits (symbols) as well as their LLRs 111. Then soft channeldecoder based on the BP decoding 105 is used to decode the soft LLRs 111into n decoded bits (symbols) 112. Finally k information bits (symbols)113 are extracted in 106. The soft inputs to the channel decoder 105could be based on channel LLRs or other equivalent metrics. The systemmay also include, not shown in FIG. 1 for simplicity, other blocks suchas source encoder/decoder, interleaver/de-interleaver, other layers ofprotocol stack, error detection encoder/decoder, radio frequencyfront-end circuitry, filters, amplifiers and one or more antennas,system clock and local oscillators. The receiver may also includeadditional blocks for carrier phase and frequency recovery,amplitude/power estimation, timing synchronization, etc. All suchadditional blocks/elements could be implemented as they are known in theart or in accordance with other embodiments.

In another scenario, a concatenated scheme is shown in FIG. 2. The innerFEC code 205 could be a convolutional code, a turbo code, an LDPC codeor any other block code. Instead of, or in addition to, inner FEC code,there may be another finite state machine representing a multiuserchannel, MIMO de-mapper/detector and similar. The system may alsoinclude other blocks as they are known in the art, as discussed above inconjunction with FIG. 1. The outer FEC code 202 could be a linear blockcode representable with a binary parity check matrix such as a BCH code,RS code, or any M/H/LDPC code. For the outer FEC code, a general case ofnon-binary code over Galois Field GF (2^(m)) is considered where eachcode symbol contains m bits and the case m=1 results in a binary code.The same assumption could be made for the inner FEC code. The input inFIG. 2 213 contains km information bits that are converted into ksymbols over GF (2^(m)) 214 in block 201. These k symbols are encodedusing an (n,k) linear block code 202 to generate n coded symbols 215that are then converted to nm bits 216 in block 203. The outer FEC codebits 216 are interleaved in 204 and the interleaved outer FEC code bits217 are encoded with an (N,K) inner FEC encoder 205. The N inner FECcode bits 218 are converted to modulation symbols 219 using the symbolmapper 206. The modulation symbols go through the channel 207 and noisymodulation symbols 220 are received at the receiver. The de-mapper 208converts the noisy modulation symbols into N inner FEC code bits LLRs221 that are next decoded using the inner FEC decoder 209. The K updatedLLRs at the output of the inner FEC decoder 222 are de-interleaved in210 and the de-interleaved code LLRs 223 are decoded using the outer FECdecoder 211 to generate decoded bits 224. Finally, information bits 225are extracted in block 212. A soft channel decoder based on BP 211 canbe used for decoding the outer FEC code, whereby soft inputs to theouter FEC decoder based on BP 211 could be provided by the inner decoderblock 209, or equivalent.

Description of Known BP Methods

In order to explain the BP decoding, the bipartite graph of the (n, k)linear block code is shown in FIG. 3 (assuming n=6 and k=3 forrepresentation). It is formed using the parity check (PC) matrix H ofthe code which is an (n−k)×n matrix for a code of rate r=k/n. In thisgraph, there are two types of nodes: (n−k) check nodes (307, 308, and309 in FIG. 3) and n variable nodes (301, 302, 303, 304, 305, and 306 inFIG. 3). For any codeword c of this code, Hc^(T)=0 where ‘T’ denotes thetranspose operation. This equation specifies the set of linearconstraints satisfied by the codeword bits. In the bipartite graph, theset of variable nodes represents the codeword bits and the set of checknodes represents the set of parity-check constraints satisfied by thecodeword bits. There is also a set of edges that connect every checknode with all the variables nodes involved in its check equation.

BP decoding is an iterative decoding method that receives the soft bits,corresponding to reliabilities of codeword bits from the channel, suchas LLRs and performs message passing (from variable nodes to check nodesand vice versa) using the bipartite graph of the code to update thereliability information based on the parity check constraints (FIG. 3).In order to explain the algorithm of BP decoding, often performed usingthe sum-product algorithm (SPA), define N_(ci) as the set of variablenodes participating in check equation i and N_(vj) as the set of checknodes that variable node j is involved with. Typical SPA implementationis summarized in the algorithm that follows immediately below, withusual variations.

Belief Propagation Decoding Based on Sum-Product Algorithm

Define two zero matrices Mvc and Mcv with the same size as the matrix H.Mvc(i,j) represents the variable to check node message from variablenode j to check node i. Mcv(i,j) represents the check to variablemessage from check node i to variable node j.

For all (i,j) such that H(i,j)=1:

1. Initialization: Mvc(i,j)=ρ(j)

where ρ(j) represents the soft output from the previous block (e.g.,channel, de-mapper or inner code) and is usually expressed in terms ofLLRs;

2. Horizontal step (check nodes updates):

$\begin{matrix}{{{Mcv}( {i,j} )} = {2\;{\tanh^{- 1}( {\prod\limits_{k \in {N_{ci}\backslash j}}\;{\tanh( \frac{{Mvc}( {i,k} )}{2} )}} )}}} & (1)\end{matrix}$where N_(ci)\j is the set of all variable nodes participating in checkequation i except for variable node j.

3. Vertical step (variable nodes updates):

$\begin{matrix}{{{Mvc}( {i,j} )} = {{\rho(j)} + {\sum\limits_{k \in {N_{vj}\backslash i}}{{Mcv}( {k,j} )}}}} & (2)\end{matrix}$where N_(vj)\i is the set of all check nodes that variable node j isinvolved with except for check node i. Mcv(k,j) represents extrinsicinformation from check node k to variable j;Steps 2 and 3 are repeated until the algorithm converges to a codewordor the maximum number of iterations is reached.

4. Output LLR:

$\begin{matrix}{{\rho_{x}(j)} = {{\rho(j)} + {\sum\limits_{k \in N_{vj}}{{Mcv}( {k,j} )}}}} & (3)\end{matrix}$based on which decisions could be made, or extrinsic information couldbe calculated in case there is feedback from the linear block codedecoder to a previous block (for example a feedback from outer decoder211 to the inner decoder 209 in FIG. 2 in order to perform iterativeconcatenated decoding).

The dashed lines in FIG. 3 show a 4-cycle in the graph meaning variablenodes 301 and 306 are both involved in check nodes 307 and 309. Thedisadvantage of short cycles is that if for example variable node 301 isin error, this error could propagate to variable node 306 during thecheck node and variable node updates.

In order to perform check node updates in the SPA, a number of tan h andinverse tan h functions needs to be evaluated. This leads to highcomplexity of implementation. Therefore, another method based on thesimplification of the SPA has been proposed which is called the min-sumalgorithm (MSA). It has been shown by experiments that the performanceof the MSA is often close to that of the SPA, yet with considerably lesscomplexity.

Min-Sum Algorithm

As it is well known in the art, the vertical step (variable node update)is similar to the one in the sum-product algorithm. The difference is inthe horizontal step (check node update) where instead of calculating allthe tan h and inverse tan h functions, a simpler, approximatealternative is used based on the shape of the tan h function. For eachcheck node, one needs to first calculate

$\begin{matrix}{{S_{ci} = {\prod\limits_{k \in N_{ci}}\;{{sign}( {{Mvc}( {i,k} )} )}}},} & (4) \\{{i_{\min} = {\arg\;{\min\limits_{k \in N_{ci}}{{{Mvc}( {i,k} )}}}}},} & (5) \\{i_{\min\; 2} = {\arg\;{\min\limits_{k \in {N_{ci}\backslash i_{\min}}}{{{{Mvc}( {i,k} )}}.}}}} & (6)\end{matrix}$Next, the check nodes are updated:Mcv(i,j)=α·S _(ci) ·|Mvc(i,i _(min))|·sign(Mvc(i,j)),j≠i _(min),  (7)Mcv(i,i _(min))=α·S _(ci) ·|Mvc(i,i _(min2))|·sign(Mvc(i,i _(min)))  (8)where α is called the damping factor. As it can be seen, the MSA onlyrequires a single floating multiplication and a modulo-2 addition (orcounting of + and − signs) to update each check node leading to muchlower complexity compared to the SPA. There are numerous approaches inthe art for implementing normalization of MSA produced extrinsicinformation, in order to reduce observed overestimation of same. Insteadof scaling/normalization by a multiplicative factor α, an additiveoffset is sometimes used in the art. Also, in certain prior art systemsthe normalization factor may be adaptive. In some prior art systems,either dumping using a scaling factor or an additive offset could beapplied in the vertical step. These approaches could be useful in bothSPA and MSA algorithms, as well as other algorithm types. Numerousattempts to improve normalization of MSA demonstrate the need to provideaccurate MSA based messages, yet with small computational complexity ofapplied normalization factors. In one aspect of the invention, a noveland simple adaptive normalization factor, tailored to the structure ofMSA check-to-variable messages.Method for Suppressing Oscillations in Variable to Check Messages

When messages from variable to check nodes alternate signs inconsecutive BP iterations, it is often an indication that variable nodeand/or part of the code graph has not converged and it might bedesirable to suppress such messages to minimize possible errorpropagation. It has been shown in the art that erasing such messageswith alternating signs is a good strategy for LDPC codes. Similarly, ithas been demonstrated in the art that taking a simple average ofvariable to check messages in two consecutive iterations, if they havedifferent signs, improves the performance of regular and irregular LDPCcodes. It would be advantageous to have a general strategy to deal withnon-converged variables for different classes of codes. In one aspect ofthe invention, a weighted average of variable to check messages in twoconsecutive iterations, if they have different signs, is employed, withmore weight given to the latest message, generally.

Informed BP Scheduling

In the original BP algorithm, all the variable nodes are updatedsimultaneously using the previous check-to-variable messages and thenall the check nodes are updated simultaneously using the previousvariable-to-check messages. This approach is often referred to asflooding scheduling and the flow of messages is illustrated in FIG. 3.

Sequential scheduling on the other hand updates the nodes sequentiallyin an effort not only to improve the convergence speed (less iterations)but also to outperform the flooding scheduling for a given number ofiterations. There are multiple ways how to implement ordering insequential scheduling which generally results in different convergencerate and/or performance.

In Informed Dynamic Scheduling (IDS), the schedule is dynamicallyupdated using the current state of the messages in the graph. ResidualBelief Propagation (RBP) performs message updates based on the absolutevalue of the difference between the message generated in the currentiteration and the message received in the previous iteration. A largerdifference means this part of the graph is further from convergence.Therefore, propagating the messages with larger differences first willmake BP converge faster. FIG. 4 shows the bipartite graph of a linearblock code with check nodes 407, 408, 409 and variable nodes 401, 402,403, 404, 405, 406 and illustrates the flow of messages and order in RBPdecoding. First, using Mvc values all the Mcv values are calculated. TheMcv(i,j) with maximum residual is selected (Mcv from check node 409 tovariable node 401 in FIG. 4) and used to update Mvc(i,j) (Mvc fromvariable node 401 to check node 409 in FIG. 4). Then Mvc(i,j) ispropagated to all the check nodes connected to it except for check nodei (409 here). In FIG. 4, this means propagation to check node 407. TheMcv values for these check nodes are calculated and used for obtainingnew residual values. The process continues by selecting the next Mcvvalue with largest residual value and propagating it.

In another method called Variable-to-Check Residual Belief Propagation(VCRBP), Mvc's with maximum residuals are propagated and used to updateMcv's. Each updated Mcv then is propagated to all the variable nodesconnected to it except the one it has received new information from.Similar to most greedy algorithms, VCRBP and RBP converge to a codewordfaster, but may converge to the correct codeword less often.

Node-wise Scheduling (NWS) is a less-greedy IDS strategy with betterperformance and convergence than RBP. In NWS, instead of onlypropagating the message with the largest residual, all thecheck-to-variable messages corresponding to the same check node areupdated and propagated simultaneously. The process is illustrated inFIG. 5 which shows the bipartite graph of a linear block code with checknodes 507, 508, 509 and variable nodes 501, 502, 503, 504, 505, 506 andthe message updating routine. First using the Mvc values, a residualvalue is calculated for every check node. The check node with maximumresidual value (509 in FIG. 5) is updated first. Then the variable nodesconnected to check node 509 (501, 504 and 506 in FIG. 5) are updated.Using the updated Mvc values, residuals for remaining check nodes areupdated and again the check node with largest residual is selected (507in FIG. 5) and is updated incorporating the newly updated variable nodes(501 and 506 here) and the process is repeated. In NWS, there are manyvariable nodes that change compared to RBP where only one variable nodechanges. Therefore, it is less likely to propagate the information fromnew errors in the next update. In both RBP and NWS, in order to pickwhich message to update, many messages are computed and not passed whichresults in high complexity of the decoding. MSA can be used to simplifythe ordering metric and significantly decrease the complexity of bothstrategies while maintaining the same performance. Still, evensimplified methods require numerous calculations of outgoing messagesthat will not be used as well as considerable complexity due to repeatedordering of residuals. Therefore, there is still a need to reducecomplexity and/or improve performance of informed dynamic scheduling fordecoding of H/M/LDPC codes. In one aspect of the invention, a simplegreedy BP algorithm is proposed with considerably smaller complexity andcomparable or better performance relative to serial scheduling, RBP andNWS approaches.

BP Decoding for H/M/LDPC Codes

Standard BP iterative decoding is not suitable for H/MDPC cods such asRS codes. The reason is that these codes have high density parity checkmatrices which results in a large number of short cycles in the factorgraph. The presence of short cycles causes correlation between themessages and results in error propagation in the graph. Adaptive BPalgorithm (ABP) is the first successful BP-based decoding method in theart for H/MDPC codes.

Methods Based on Extending the Parity Check Matrix

There were several attempts to extend the parity check matrix in orderto minimize the number of short cycles that adversely affect theperformance of BP decoding. Some of these methods showed relativesuccess in case of short codes but no evidence of improvement for longcodes. In another method proposed in the art for decoding of longercodes, RS codes, the binary parity check matrix can be extended withrows and columns in order to reduce the number of short cycles. Thismethod has affordable complexity but the performance is not nearly asgood as the ABP algorithm described below.

Methods Based on Multiple Parity Check Matrices

In certain prior art systems, multiple random matrices are employed overiterations, for improved decoding of short codes with dense parity checkmatrix. Improved decoding of high density parity check matrices forshort codes has also been demonstrated in the art by employing multiple,fixed, parity check matrices. No such improvements were shown for longercodes. There is a need to improve upon the performance of these methodsand especially for longer codes. In one aspect of the invention,multiple suitably chosen parity check matrices are employed. Thematrices are selected based on certain reliability criterion and furtheradapted over iterations.

Adaptive BP Algorithm

In the adaptive BP algorithm of a (n,k) linear block code, the LLRs ateach iteration are used to divide the variable nodes into two groups,(n−k) low reliable (LR) nodes and k high reliable (HR) nodes. Thecolumns of the binary parity check matrix corresponding to the (n−k)independent and least reliable bit positions are reduced to an identitysubmatrix. The BP iteration is then performed on the factor graph formedby the modified parity check matrix. Since (n−k) least reliable bits arenot involved in any cycles, error propagation is limited. Also, thesebits only receive extrinsic information from one check node only whichis based on the reliability of the other bits connected to this checknode which are expected to be correct. Therefore, the performance of BPdecoding is improved. During the p-th iteration, first the parity checkmatrix is reduced to the desired form based on the LLR vector L_(p)(initially L₀ is determined from the channel output). In the secondstep, BP decoding is applied to produce extrinsic LLRs L_(p−ext). (whereL_(p−ext)(j)=Σ_(kεN) _(vj) Mcv(k,j)). The updated reliabilities are thenL _(p+1) =L _(p) +αL _(p−ext)  (9)where 0<α≦1 is a damping coefficient.

In the original ABP method, after each BP iteration, a decoder isapplied on the updated reliabilities. This decoder could be a simplehard decision application or any existing decoder for the H/M/LDPC code.

For example, for RS codes, this decoder could be one of the following:

Hard Decision: Perform hard decisions on the updated LLRs. If theresults satisfy the parity check equations, then a decoding success issignaled.

BM: Run the Berlekamp-Massey (BM) algorithm on the LLRs after harddecisions. If the BM algorithm finds a codeword, a decoding success issignaled.

KV: Run Koetter-Vardy (KV) soft decision decoder on the LLRs. KVcombined with the ABP algorithm results in impressive coding gains overpreviously known soft decision decoding algorithms for RS codes.However, KV soft decision decoders are characterized with extremely highcomputational complexity relative to BM algorithm. There is a need tohave better decoding in this step than with BM algorithm but withsmaller complexity than with KV algorithm. In one aspect of theinvention, unreliable symbols are estimated/flagged and used for erasureBM decoding to yield improved performance over BM decoding.

The stopping criterion in ABP algorithm is when a decoding success issignaled by the decoder or the maximum number of iterations is reached.The final performance depends on the type of the decoder used after eachBP iteration.

Best Graph Algorithm

The best graph Algorithm (BGA) is based on the same idea of modifyingthe graph of the code at each iteration to make it suitable to BP, i.e.,to place less reliable variables in the sparse part of the parity checkmatrix. The difference between ABP and BGA lies in the method by whichthe bit reliabilities are updated after the graph has been modified. Inthe p-th iteration, the original H is reduced based on the LLR vectorobtained in the previous iteration. BP is then performed on this newmatrix using correct BP message passing. Because the matrix H at eachiteration is different, one needs to find elements of Mvc for the newmatrix. In order to obtain the new Mvc values, the following rule isused:

Complete condition: For Mvc(i,j), from the previous Mvc matrices themost recent one with non-zero Mvc(i,j) is found such that at the sametime the variable node j was a high reliable (HR) node. However, ifthere has never been a connection between check node i and variable nodej or the previous connection corresponds to the case where the variablenode j was a low reliable (LR) node, the input LLR of the jth variableis used as Mvc(i,j).

In BGA, in order to prevent error propagation, at any iteration one canalways use the input LLR of the jth variable node for Mvc(i,j) with acertain probability Proba. Therefore, the above condition is only usedwith probability (1-Proba). This probability depends on the probabilityof creating a “pseudo-loop” by selecting a previous Mvc(i,j) and can beadjusted using simulations. In spite of improvements, in some cases,over ABP method, BGA still desires better performance especially forlonger codes. In one aspect of the invention, BGA decoding is improvedby using a suitable choice of multiple representations of parity checkmatrix based on a reliability criterion.

Description of Inventive Aspects Pertaining to H/M/LDPC Decoding

Novel Adaptive Normalized Min-Sum Algorithm

In some embodiments, an adaptive normalized MSA algorithm is employed.Preferably, a simple scaling factor adaptation is tailored to MSA. Thenovel adaptive normalized scaling factor improves MSA performance.

Damping the check to variable messages using a scaling factor couldreduce the overestimation of extrinsic values and could consequentlyhelp to improve the performance of the min-sum algorithm as shown in theprior art. The scaled check to variable message could be represented asMcv(i,j)=δ(i,j)*Mcv(i,j)  (10)where simple adaptive scaling factor is given by:

$\begin{matrix}{{\delta( {i,j} )} = {1 - {\beta\frac{{Min}_{1}}{{Min}_{2}}}}} & (11)\end{matrix}$where 0≦β≦1 can be determined by experimentation/simulations for aparticular code. Min₁ and Min₂ are the two smallest values in the set{|Mvc(i,:)|\|Mvc(i,j)|}. The scaling factor using the above proposedmethod can be calculated for all the variable nodes connected to checknode i easily with a very affordable complexity using only the threesmallest values of the set {|Mvc(i,:)|}. Denoting these three smallestabsolute values of variable to check messages by L1, L2 and L3 andassuming L1 and L2 correspond to variable nodes jmin1 and jmin2, simpleadaptive scaling factor can be written as

$\begin{matrix}{{\delta( {i,{j\;\min\; 1}} )} = {1 - {\beta\frac{L\; 2}{L\; 3}}}} & (12) \\{{\delta( {i,{j\;\min\; 2}} )} = {1 - {\beta\frac{L\; 1}{L\; 3}}}} & (13) \\{{{\delta( {i,j} )} = {1 - {\beta\frac{L\; 1}{L\; 2}}}},\mspace{14mu}{j \neq {j\;\min\; 1}},{j\;\min\; 2.}} & (14)\end{matrix}$such that 0≦δ≦1. The numerator in above equations also corresponds tothe absolute value of the check to variable messages from the check nodei. Thus when |Mcv(i,j)| is larger, the corresponding δ(i, j) is smaller,and vice versa. That is, the scaling factor is set to suppress morelarger-messages and less smaller-messages in an adaptive manner. Thedenominator in above equation is always larger than or equal to thenumerator and represents a rough measure of the reliability of remainingvariables connected to the corresponding check node. Thus if remainingvariables of the same check equation are more reliable, less suppressionof extrinsic information is done, and vice versa. In another embodiment,the adaptive scaling factor could also be used in SPA. In yet otherembodiments, the adaptive scaling factor could be used in other suitablevariations of BP algorithms. The performance gain of adaptive normalizedMSA (ANMSA) of certain aspects of the invention, compared to normal MSAon the example of RS (255,239) code is shown in Table 1.

TABLE 1 BER performance of Novel Adaptive Normalized MSA (NANMSA). Eb/No(dB) 6.5 7.0 7.5 7.75 MSA 0.007 0.004 0.002 0.001 ANMSA 0.0043 0.00140.00047 0.0002Weighted Averaging of Non-Converged Variable to Check Messages

In another embodiment, for some codes it might be advantageous to employa weighted average of variable to check node messages if such messageshave different signs in consecutive iterations. Specifically, if thesigns of Mvc(i,j) messages are different in iterations n and n−1, thenthe modified Mvc(i,j) is given by:{tilde over (M)}vc(i,j)^((n)) =g×Mvc(i,j)^((n))+(1−g)×Mvc(i,j)^((n-1)),where 0.5≦g≦1, thus giving some preference (more weight) to the newermessage, though still suppressing potential propagation of “bad”message. Table 2 shows the performance improvement over ANMSA indecoding RS (255,239) code when averaging of variable to check messagesis employed. g=0.5 case corresponds to equal weight averaging in theprior art, while opt. g corresponds to the approach according to thisembodiment, that is when weighting factor is optimized. It could be seenthat the aspect of the invention in this embodiment, with optimizedweighted averaging, further improves the performance over prior artmethods, with essentially same computational complexity.

TABLE 2 BER Performance improvement due to averaging of Mvc. Eb/No (dB)6.5 7.0 7.5 7.75 ANMSA 0.0043 0.0014 0.00047 0.0002 ANMSA, g = 0.5 0.0030.00046 0.00007 0.000011 ANMSA, opt. g 0.002 0.00023 0.00003 0.0000052Simple Greedy BP Scheduling

This method is an attempt to perform informed BP decoding withoutcalculating all the residual values in every iteration. The main idea isto first update the equations that are less likely to have errors in thedense part of the parity check matrix. For each check node i, only onevalue is calculated and assigned based on only outgoing messages. Thisvalue is the sum of the two smallest values in the set {|Mvc(i,:)|}. Inany iteration of BP decoding, the proposed simple greedy scheduling hasfour main steps:

-   -   1. For each check node, calculate a scheduling metric value:        Val _(i)=Min₁+Min₂  (15)        where Min₁ and Min₂ are the two smallest values in the set        {|Mvc(i,:)|}. These values (Val's) are sorted in a decreasing        order and an ordering vector I={I₁, I₂, . . . , I_((n-k))} is        determined and saved.    -   2. Perform check-to-variable updates using a greedy algorithm        based on the ordering from step 1. The process is similar to the        one shown in FIG. 5. A check node is selected based on ordering.        All the Mcv messages corresponding to that check nodes are        updated. The changes in the Mcv values are used to update the        Mvc messages corresponding to variable nodes connected to that        check node. Then the next check node is selected and its Mcv        values are updated using the most recent Mvc values. The process        is shown in the following steps:    -   a) Inputs: previous variable-to-check and check-to-variable        messages, Mvc_(old) and Mcv_(old).    -   b) Initialization: define a zero matrix Del of the same size as        the binary parity check matrix. This matrix shows the changes in        the Mcv values. These changes are used to update the Mvc values        subsequently.    -   c) Start from the check node corresponding to the first position        I₁ in the ordering vector I of step 1. Use normalized MSA with        parameter β (alternatively another algorithm known in the art        may be used) to update all the check-to-variable messages of        check node I₁. Also, preferably, multiply check-to-variable        messages with a scaling constant α. The new updated messages are        represented by Mcv_(new)(I₁,:).    -   d) Update matrix Del as:        Del(I ₁,:)=Del(I ₁,:)+Mcv _(new)(I ₁,:)−Mcv _(old)(I ₁,:)  (16)    -   e) Update the Mvc values:        Mvc _(new)(I ₁,:)=Mvc _(old)(I ₁,:)+Del(I ₁,:)  (17)    -   f) Repeat the above procedure c)-e) for next positions of the        ordering set I.    -   3. Perform variable-to check updates:

$\begin{matrix}{{{Mvc}_{new}( {i,j} )} = {{\rho(j)} + {\sum\limits_{k \in {N_{vj}\backslash i}}{{Mcv}( {k,j} )}}}} & (18)\end{matrix}$

In order to improve this part, use the following rule. In cases wherethe sign of a variable-to-check message changes from the previousiteration, an average between the two messages may be computed:Mvc _(new)(i,j)=g·Mvc _(new)(i,j)+(1−g)·Mvc _(old)(i,j)  (19)where 0.5≦g≦1 determines the weight that is given to each message.

-   -   4. Using the updated Mvc's, it is possible to determine which        one of the parity check equations are satisfied at the end of        the iteration. The number of satisfied checks is measured in        this step. It can be seen that when all the checks are        satisfied, the algorithm has converged to a codeword.

This method requires a lot less computations compared to RBP and NWS,yet provides excellent performance. It does not need to calculateresiduals after each check node update, for all the remaining unupdatedcheck node equations, in order to choose which check node to updatenext. Therefore, it avoids calculating unused residual updates fordetermining the order of check node updates and correspondingly reducesthe complexity of residual calculations on average by approximately V·

${V \cdot \frac{N^{2}}{2}},$where N is the number of check equations and V is the average number ofvariable nodes connected to a check node. The assigned values to thecheck nodes are only calculated and sorted once at the beginning andcheck node schedule starts from the check node with maximum value Valand continues according to the calculated ordering vector. Alsocalculating Val is very easy because it only depends on the outgoingmessages and no actual check node updates are required. This results ina very affordable complexity.

In Table 3, the performance of simple greedy algorithm is compared withthe min-sum algorithm as well as the normalized min-sum algorithmincluding weighted averaging of non-converged variable to checkmessages. The latter is referred to as Min-Sum 2D algorithm in thetable. The performance is also compared with the serial scheduling whichis similar to the simple greedy except that step 1 is not performed andcheck node updates are performed from the first check node to the lastone without any additional ordering meaning I={I₁, I₂, . . . ,I_((n-k))}={1, 2, . . . , (n−k)}. The comparison is performed for RS(255,223) and 3 iterations for each decoder. As it can be seen in Table3, the proposed simple greedy algorithm has the best performance.

TABLE 3 Comparison of simple greedy algorithm with other BP decodingtechniques. Eb/No (dB) 6 6.5 7 7.5 Min-Sum FER 1 0.86 0.64 0.32 BER1.45e−2 1.29e−2 1.16e−2  8.1e−3 Min-Sum 2D FER 0.96 0.75 0.34 0.096 BER  8e−3   7e−3  4.9e−3  1.9e−3 Serial FER 0.909 0.51 0.101 0.0112Scheduling BER 3.98e−3 1.49e−3  2.3e−4 2.46e−5 Simple Greedy FER 0.90.438 0.0788 0.00749 BER 3.95e−3 1.48e−3 2.15e−4 1.89e−5Alternative Simple Greedy BP Algorithm

In another embodiment, in step 1 of the simple greedy algorithm, insteadof sorting all the values, Val's, only the check node with maximum Valis determined. Then in step 2, check-to-variable updates are performedfor the check node indicated in step 1. Then step 3 is performed asbefore. After that going back to step 1, only the index of the row withmaximum Val among remaining un-updated check equations is determined.Then in step 2 check-to-variable updates are performed for the newlydetermined check node followed by step 3. Going back to step 1, the nextcheck node with maximum Val is selected and the process continues untilall the check nodes are updated. Step 4 will be similar as before. Thus,instead of sorting complexity of N values, Val, initially, withcomplexity proportional to N·log N, in this alternative approach, beforeeach check node update, a maximum value of Val for the set of remainingunupdated check nodes is found. This results in computational complexityproportional to N·(N−1)/2. In some cases, this alternative approachyields slightly better results.

Improved Best Graph BP Using Multiple Parity Check Matrices

A soft decoding algorithm for H/M/LDPC codes based on the beliefpropagation method has been invented. The method is based on BGalgorithm extended with multiple parity check matrices. The performanceimprovement of both ABP and BGA methods is due to the fact that thecolumns corresponding to low reliable bits are sparsified preventingerrors to be propagated specially for M/HDPC codes. Here, some of themethods described add diversity to BGA by starting with the main paritycheck matrix of the code and generate multiple matrices with differentsparsified parts. This way, even some of the lower reliable bits in khigh reliable (HR) group will also be put in the sparse parts of one ofthe matrices. This way error propagation from higher reliable bits mayalso be prevented. This will help to improve the final performance intwo ways. First, it allows for errors in bit positions that wouldnormally be in the dense part of the parity check matrix in ABP and BGmethods. Secondly, multiple matrices provide a “decoding diversitymethod” in that one matrix may enable convergence to the solution whileothers may not, which indeed has been confirmed by extensivesimulations. The steps in the decoder are explained in the following.Also the main steps 1-7 are shown in FIG. 6:

Step 1: The input bit LLRs 608 are sorted in 601 based on their absolutevalues. The resulting index vector 609 is called I_sort.

Step 2: In BP decoding, one should use the binary parity check matrix ofthe code. In ABP, the columns of the H matrix corresponding to (n−k)least reliable bit positions (LRBs) are converted to degree 1 columns.This way the errors in low reliability bits are prevented frompropagating to “healthy” bits in the dense part. In order to improve theperformance even more, diversity can be added by generating N_matmatrices with degree 1 columns in different positions (FIG. 6, 602):

-   -   1st: Modify H by converting its (n−k)−L columns corresponding to        least reliable bits to degree 1. Call the new matrix Hp.    -   2nd: For v=1:N_mat, start with Hp and convert L columns        corresponding to I_sort((n−k)−L+(v−1)L−1: (n−k)−L+vL) to degree        1 and call the resulting matrixH_(v). In the end, there will be        N_mat matrices H₁, H₂, . . . , H_(N) _(_) _(mat) in lines 610,        611, . . . , 612. Row additions are used to make the degrees of        selected columns equal to one. All of the matrices have a common        set of (n−k)−L columns with degree 1, corresponding to lowest        reliable bits. In addition, each of the matrices has another        different set of L columns with degree 1. The position of these        L columns are different for each matrix providing the matrix        diversity and allowing different higher reliable bits to be in        the sparse part, in case errors occur in these more reliable        positions.

The effect of matrix diversity explained above, number of iterations foreach matrix and the value L explained above has been investigated for RS(255,239) in FIG. 7. From the two right curves, the performanceimprovement of using BM decoder at the end of iterations is evidentcompared to only using the hard decisions of the updated LLRs. As it canbe seen, by increasing the number of matrices, frame error ratedecreases. Both of the two curves at the left use 10 matrices but one ofthem performs 2 simple greedy BP iterations for each matrix while theother one performs 10 iterations resulting very slightly betterperformance. The simple greedy BP algorithm does not need manyiterations to reach good performance. The value of L should be adjustedfor each code using simulations. For example, for RS (255,239) code,L=16 provides the best results.

Step 3: For each of the N_mat matrices, an Mvc matrix is defined basedon the input reliabilities 615 such that: Mvc(i,j)=ρ(j), for all (i,j)where H_(v) (i,j)=1 and Mvc(i,j)=0, for all (i,j) where H_(v)(i,j)=0.The process for H₁ is shown in FIG. 6 where BGA block 603 is used toform the Mvc₁ matrix 616 using H₁ in line 614 and input LLRs in line615. For the first iteration, there is no input in line 621 becausethere are no saved Mvc matrices.Step 4: For each of the N_mat matrices generated in the last step,Bpit_in number of BP iterations are performed using the proposed simplegreedy BP algorithm in block 604. Alternatively, other BP algorithmscould be used. This way, a set of N_mat updated bit reliabilities,Lt_(v), v=1:N_mat are generated. The updated LLR for H₁, Lt₁, is shownin line 617. All the N_mat final Mvc matrices for all the N_mat casesare saved. The final Mvc₁ for H₁ is shown in line 620 which is saved inblock 607.Step 5: For v=1:N_mat, Lt_(v) vector is sorted based on its absolutevalues (FIG. 6, 605) which results in the index vector I_v shown in line618 for H₁. Then H_(v) is modified such that the columns correspondingto I_v(1:(n−k)−L) and I_v(((n−k)−L+(v−1)L+1:(n−k)−L+vL) are converted todegree 1. Block 606 receives H₁ 619 and its corresponding index 618 andgenerates the modified new H₁ 613. It should be noted that a lot ofthese columns already have degree 1. Thus, one may only need to convertto degree 1 those columns that are not of degree 1. In the end, therewill be N_mat modified matrices.Step 6: In order to be able to continue the correct BP iterations foreach of the N_mat cases, new updated Mvc matrices corresponding to eachof the modified N_mat parity check matrices need to be calculated. Theprinciples of the best graph algorithm are used for this task. For eachcase, the new matrix (613 for H₁), previously saved Mvc matrices (621for H₁) and input LLRs 615 are used by the BGA algorithm in block 603 toobtain the new Mvc matrix (616 for H₁). According to certain aspects ofthe invention, it is proposed that instead of the complete conditionexplained in the discussion of BGA above, the following simple conditioncan be used without noticeable performance loss:

-   -   Simple condition: For Mvc(i,j), from the previous Mvc matrices,        the most recent one with non-zero Mvc(i,j) is found and its        value is used for the new Mvc(i,j). However, if there has never        been a connection between check node i and variable node j, the        input LLR of the jth variable is used as Mvc(i,j).

Similar to the complete condition, the input LLR of the jth variablenode can always be used for Mvc(i,j) with a certain probability Proba.The above condition is only applied with probability (1-Proba). Based onextensive examples and simulations, for longer HDPC codes, such as RS(255,223), it may be better to use the input LLRs for new Mvc valuesrather than the BGA, meaning Proba=1.

Step 7: Steps 4-6 are repeated Bpit_max times or until the iterationswith at least one of the matrices converge to a valid RS codeword.

Step 8: In this step, for each of the v=1:N_mat matrices, there existseither a decoded RS codeword or a set of updated LLRs, Lt_(v). Thecodewords are saved in a list. For cases that have not converged to acodeword, the updated LLRs, Lt_(v) are decoded by a known hard decisionor some kind of erasure or soft decision decoder for that code. If acodeword is decoded, it will be added to the list of codewords. Theinput LLRs and the average of Lt_(v)'s are also decoded by a known harddecision decoder or some kind of erasure or soft decision decoder forthat code.Step 9: If multiple codewords exists in the list, the one with minimumEuclidean distance from the received signal is chosen. However, when thedecoder fails to generate even one codeword, one can select the mean ofN_mat LLR vectors from BP iteration or simply select the input LLRs asthe output of the decoder.Proposed Decoder Using Other BP Algorithms

In other embodiments, in step 4 of the above proposed algorithm, insteadof the simple greedy BP algorithm, one can use any of the other BPdecoding methods such as flooding BP (SPA, MSA, normalized MSA, etc.) aswell as informed BP scheduling (RBP, NWS, etc.).

Choosing the Best LLR

In step 9, when the decoder fails to generate even one codeword, one canuse input LLRs, average of N_mat LLRs or one of the N_mat LLRs as theoutput of the decoder. Extensive experiments have been performed inorder to choose the best possible LLR vector in order to reduce thefinal bit error rate. All the output LLR sets of BP decoderscorresponding to N_mat different matrices were considered, as well asthe average of these N_mat LLR sets and also the input LLRs. From theexperiments with long RS codes, it appears that the LLRs from the firstmatrix and also the average LLRs generally yield the best bit errorrate, though the difference is very small. More systematic measurementsof the performance suggest that mean of LLRs from multiple matricesresults in overall best performance.

Error and Erasure Decoding at the End of BP Iterations

In another embodiment, in step 8, the final decoder applied to the LLRsof not converged matrices can be improved using error and erasuredecoding. As an example, for RS codes, the final decoder may be a harddecision BM decoder or a soft decision KV decoder. BM decoder is muchsimpler than the KV decoder but its performance is worse. Therefore, thewhole decoder based on BP will have much better performance using the KValgorithm but with a very high complexity. Erasure decoding is anattempt to have better performance compared to a hard decision decodersuch as BM decoder and less complexity compared to a soft decoder suchas KV. According to this embodiment, first the updated LLRs, Lt_(v) areused to determine a set of unreliable symbol positions that if erasedmight help the BM decoder to decode successfully. Potential erasuresymbols could be identified based on probability of symbols beingcorrect. In one embodiment, a certain number, NE<=NEmax, of leastreliable symbols could be flagged for erasures, where NEmax is themaximum possible number of erasures for that particular code. In analternate embodiment, all symbols with probability of correct smallerthan a threshold could be erased, such that the number of erasures doesnot exceed NEmax. After selecting the erasures, the hard decisions ofeach set of reliabilities along with the erasure positions are appliedto the BM error and erasure decoder.

Proposed Decoder Using Disagreement Positions

In another embodiment, the proposed decoder can take advantage of thefact that the sign disagreement positions between the N_mat LLR vectors,Lt, have a very high probability of being actually bit errors. It hasbeen observed by extensive experimentation that approximately half ofdisagreement positions correspond to errors. Therefore, placing thesesign disagreement positions in the sparse part of the matrix might helpto prevent error propagation from these positions and consequently toimprove the final performance. Using this observation, in thisembodiment, the step 5 of the proposed algorithm can be modified as

Step 5: For the N_mat LLR vectors Lt generated in the last step, thesign disagreement positions between all of them are found. The vector ofselected positions is called I_dis. For v=1:N_mat, Lt_(v) vector issorted based on its absolute values which results in the index vectorI_v. Then H_(v) is modified such that the columns corresponding toI_dis, I_v(1:(n−k)−L) and I_v(((n−k)−L+(v−1)L+1: (n−k)−L+vL) areconverted to degree 1 in the same order mentioned. Depending on the rankof the matrix, it might not be possible to convert all the mentionedcolumns to degree 1 and some of the last positions mentioned might havedegrees larger than 1 (e.g., 2, 3, etc.). It should be noted that a lotof these columns have degree 1 already. So, one only needs to convertthose columns that are not degree 1. In the end, N_mat modified matricesare calculated where in all of them the disagreement positions areplaced in the sparse part.

For RS (255,239), the performance of the proposed decoder is comparedwith its alternative using disagreement positions in Table 4. In bothmethods, 7 rounds of sorting and matrix adaptation are used. During eachround 3 iterations of simple greedy BP algorithm are performed exceptfor the last round where 9 iterations are performed. At the end ofiterations for each matrix, BM error and erasure decoding is used. Thenumber of matrices is set to N_mat=6. As it can be seen from this table,it is not clear which of these two methods is absolutely gives betterperformance. Based on this observation, a new decoder is proposed in thefollowing.

Combination of the Original Proposed Decoder and the AlternativeEmbodiment with Disagreement Positions

In another embodiment, first the original proposed decoder is applied.If it failed, the alternative with disagreement positions is applied.For RS (255,239), the performance of this proposed decoder is also givenin Table 4 which clearly outperforms the first two decoders especiallyat higher SNR values. Specifically, it could be seen that the combineddecoder of this embodiment reduces BER and FER several times withrespect to either of the two individual decoders. It should also benoted that the combined decoder has only slightly higher complexity thandecoder 1 (Proposed decoder) at high SNR, because decoder 2 (Proposeddecoder using disagreements) is only invoked if decoder 1 fails.

TABLE 4 Comparison of the performances of the proposed decoder with itsalternative using disagreement positions as well as a method using theircombination. Eb/N0 (dB) 4.5 5 5.5 1 Proposed decoder FER 0.31 0.05240.00095  BER 3.6 × 10⁻³ 4.6 × 10⁻⁴ 6.6 × 10⁻⁶ 2 Proposed decoder FER0.32 0.044  0.0013  using disagreement BER 3.7 × 10⁻³ 3.7 × 10⁻⁴ 8.3 ×10⁻⁶ positions 3 Combination of 1 and 2 FER 0.31 0.032  0.000278 (run 2when 1 fails) BER 3.9 × 10⁻³   3 × 10⁻⁴ 2.1 × 10⁻⁶Decoding RS Codes Using the Proposed Method

In another embodiment, the performance of the proposed methods describedin the previous embodiments is investigated for RS codes. RS codes arenon-binary linear block codes. The first step is to derive a binaryparity check matrix for a given RS code. For a RS code defined overGalois Field GF (2^(m)), the primitive element a has a companion m×mbinary matrix c. Any other non-zero element of GF (2^(m)) can be writtenas α^(s), 0≦s≦2^(m)−2 with a companion m×m binary matrix of the formc^(s). Therefore, all the non-binary elements of the (n−k)×n paritycheck matrix can be replaced with m×m binary matrices which results in a(n−k)m×nm binary parity check matrix. After finding the binary paritycheck matrix, all the discussed methods of previous embodiments areapplicable to RS codes. In FIG. 8, the performance of the proposeddecoder based on the combinations of the original proposed decoder andthe one with disagreement positions is compared with the other existingdecoding methods for RS codes in the literature for RS (255,239).Similar to the results shown in Table 4, the number of matrices is setto N_mat=6. 7 rounds of sorting and matrix adaptation are performed with3 iterations of simple greedy BP algorithm for each round except for thelast round where 9 iterations are performed. At the end of iterationsfor each matrix, BM error and erasure decoding is used. Therefore, theproposed method consists of 6×7=42 sorting and matrix adaptations aswell as 6×27=162 BP iterations using the simple greedy algorithm. At theend of iterations a maximum of 6+2=8 (6 matrices, input LLRs and theaverage of all LLRs from 6 matrices) BM error and erasure decodings areperformed, if BP decoders did not converge to a codeword duringiterations. These numbers are occasionally doubled when the originalproposed decoder fails and the method with disagreement positions needsto be performed. The method based on the combination of the ABPalgorithm with KV decoding proposed by El-Khamy (ABP-ASD, #20*50) hasthe best performance in the literature. This method performs 50 roundsof decoding each with 20 ABP iterations. In each round a different setof bit positions are converted to degree 1 in the parity check matrix.Also, after each ABP iteration, KV decoding is performed on the updatedLLRs. Therefore, this method needs 50×20=1000 sorting and matrixadaptations as well as BP iterations and KV decodings. The dominant partin the complexity of the KV algorithm is the interpolation part with atime complexity of 0(n²λ⁴) where λ is a complexity parameter determinedby the interpolation cost. The performance of the KV algorithm improvesby increasing the value of λ. The high complexity of KV makes itinapplicable in most practical scenarios. The BM algorithm has a timecomplexity of o(n(n−k+1)). As it can be seen form FIG. 8, the proposedmethod according to certain aspects of the invention outperformsABP-ASD, #20*50 of El-Khamy with about 0.15 dB with significantlysmaller complexity. In terms of sorting, matrix adaptations and BPiterations, the proposed methods are at least 1000/(2*42)≅12 timessimpler than ABP-ASD, #20*50. In addition, the proposed method uses 8 BMerror and erasures decodings while ABP-ASD, #20*50 performs 1000 KVdecodings, resulting in approximately

$\frac{1000*255*\lambda^{4}}{8*( {255 - 239 + 1} )}$times smaller decoding complexity for this part.II. Advanced Channel State Information Estimation

This subsection discusses techniques for performing advanced channelstate information, also referred to as CSI, estimation. Specifically,certain aspects of the invention discussed herein pertain to enhancingpilot channel estimation by using an optimum filter length, adaptivedecision-directed channel estimation and/or performing the estimationiteratively using shorter filters. These aspects are applicable to HDRadio systems (e.g., to improve receiver performance given that a betterunderstanding of channel attributes can be ascertained and used to adapttransmission to channel conditions), as well as to any othercommunication system where channel response may change over time, suchas various broadcast or mobile cellular systems. In some embodiments,the pilot structure is also adapted in the transmitter to properly matchchannel selectivity, thereby enabling better CSI estimation in thereceiver.

CSI estimation typically includes estimation of channel-induced phaseand amplitude on received symbols (which may be referred to as thechannel response) as well as noise power estimation. These attributesare used for different processing tasks in communication receivers, suchas coherent demodulation, diversity combining, FEC decoding and othersas known in the art. Accurate estimation of CSI is very important toproperly facilitate these processing tasks and achieve maximum-possibleperformance gains offered by those processing tasks. To facilitate CSIestimation, pilot symbols are inserted in a stream of data symbols. FIG.9 shows typical arrangements of pilot symbols in single carrier systems.In some code-division multiple-access (CDMA) systems, a dedicated pilotchannel 2101 is used to carry a continuous stream of pilot symbols 2102as shown in FIG. 9a . In other systems, pilot symbols could be clusteredand occupy a fraction of packet, such as a preamble 2103 as illustratedin FIG. 9b or a midamble 2107 in GSM systems as illustrated in FIG. 9c .In other systems, one or more pilot symbols 2113, 2121 could beinterleaved with data 2114, 2120 as shown in FIG. 9d and FIG. 9 e.

In multicarrier systems, such as orthogonal frequency-divisionmultiplexing (OFDM), pilot symbols can be arranged in time and frequencydomains. FIG. 10 illustrates typical arrangements of pilot symbols inmulticarrier systems. As shown in FIG. 10a , a select set of OFDMsubcarriers are dedicated to pilot symbols 2132 while other subcarriersare dedicated to data symbols 2131. FIG. 10b shows another arrangementin which a select set of subcarriers carry both pilot 2143 and data 2144symbols interleaved while other subcarriers carry only data symbols. Inaddition, pilot symbols may be staggered in time on two adjacentsubcarriers 2145, 2146 that carry pilot symbols. Pilot symbols are alsoknown in the art as reference symbols or training symbols.

CSI Estimation Based on Received Channel Symbols

In some embodiments, certain aspects of the invention are applicable tosystems where no iterative channel estimation is used. The correspondingmethods could also be used as initial CSI estimation in systems whereiterative, joint CSI estimation and FEC decoding are employed. Althoughcertain aspects described herein generally apply to various pilotarrangements shown in FIG. 9 and FIG. 10, specifics of implementationwith some details may differ for different arrangements. Consider firstpilot symbol arrangements with continuous pilot symbols shown in FIG. 9aand FIG. 10a . Received pilot symbols on the pilot channel in FIG. 9a oron one pilot subcarrier in FIG. 10a could be mathematically modeled asr(i)=h(i)p(i)+n(i) for i=1,2  (20)where h(i) represents the channel response, p(i) is the transmittedpilot symbol, n(i) is the additive white Gaussian noise (AWGN) with zeromean and variance. E(|(n(i))|²)=σ_(n) ². The channel response could bewritten as h(i)=a(i)e^(iθ(i)) where a(i) and θ(i) represent theamplitude and phase of the channel response, respectively.

By multiplying the received symbols with p*(i), we obtainy(i)=r(i)p*(i)=h(i)+n′(i), for i=1,2  (21)where noise {n′(i)} has the same statistics as {n(i)} and |p(i)|=1 isassumed without loss of generality. The sequence {y(i)} is used toestimate channel response and noise power. In multicarrier systems (MC),(21) could be rewritten asy _(k)(i)=r _(k)(i)p _(k)*(i)=h _(k)(i)+n _(k)′(i), for i=1,2  (22)where subscript k denotes the k-th subcarrier of the MC system.

As can be seen from (21), the i-th received sample y(i) represents thechannel response distorted by noise. Additive noise impact on thechannel estimation could be reduced by use of a finite impulse response(FIR) smoothing filter. For example, assuming the channel response isconstant over N consecutive samples, where N is an odd number, themaximum likelihood estimate of the l-th channel is given by

$\begin{matrix}{{\hat{h}(l)} = {{\frac{1}{N}{\sum\limits_{i = {1 - \frac{N - 1}{2}}}^{l + \frac{N - 1}{2}}{y(i)}}} = {{h(l)} + {\frac{1}{N}{\sum\limits_{i = {l - \frac{N - 1}{2}}}^{l + \frac{N - 1}{2}}{n^{\prime}(i)}}}}}} & (23)\end{matrix}$where a rectangular (i.e., constant filter-tap gains) FIR filter isused. By increasing the filter length N, the impact of noise is reducedand the channel estimation error is reduced. However, the filter lengthdepends also on the rapidity of channel variations. FIG. 11a -FIG. 11cshow typical examples of slow, moderately fast (or medium-speed), andfast fading channels, respectively. The length N of the smoothing filtershould be selected such that the channel changes are insignificant overthe length of the filter. Otherwise, the smoothing over an excessivenumber of samples corresponding to a varying channel response will causean estimation error due to the mismatch between channel variations andthe filter length. This tradeoff is illustrated in FIG. 12 for theexamples of slow 2151, moderately fast 2152, and fast 2153 fadingchannels shown in FIG. 11. The variance of estimation error has twocomponents, one due to the additive noise 2163 and the other due to themismatch between the filter length and rapidity of channel variation,i.e., channel dynamics 2167 or also referred to as time selectivity inwhich variance errors due to the mismatch are shown in three curves,namely, for slow fading 2166, moderately fast fading 2165, and fastfading 2164. In general, as shown in (23), the variance due to noise isdecreased by increasing the filter length N. On the other hand, thevariance of estimation error may increase by increasing N beyond therange over which the channel changes are insignificant. Therefore, for agiven rate of channel variation, i.e., time selectivity, there is anoptimum filter length that minimizes the overall impact of noise andmismatch between filter length and channel dynamics. If the channel isslowly varying, longer filter lengths will decrease the mean squareerror (MSE) of the channel estimation. On the other hand, if a longerfilter length is selected for fast fading channel, it will causeundesirable performance degradation. Therefore, the tap length selectionmust be done properly at the receiver depending on the channel dynamics.As illustrated in FIG. 12, the optimum filter length is chosen such tominimize the total variance of estimation errors due to noise andmismatch, e.g., N₃ for fast fading, N₂ for moderately fast fading, andN₁ for slow fading, respectively, where N₃<N₂<N₁. Clearly, a shorterfilter length is desired when the time selectivity of the channelincreases.

In some embodiments, non-rectangular filters could be used to give moreweight to samples closer to the current sample and less weight tosamples further apart. In the presence of impulsive noise, a medianfilter, possibly with additional smoothing, may be preferable tominimize the impact of such impulsive noise. In yet another embodiment,a new filtering method that is robust in the presence of impulsive noisecould be employed. For example, the samples with strong noise could befirst identified, say, by using median filtering and smoothing. That is,samples that significantly deviate from the smoothed median channelresponse are identified as samples corrupted by impulsive noise. Aftersamples with strong noise are identified and removed (e.g., with anamplitude of zero assigned to those samples), optimal filtering could beapplied to the remaining, non-uniformly spaced samples in the sequence.Alternatively, instead of removing those identified as samples withstrong noise, the median value of the remaining samples within thefilter length could be used before proceeding with optimal filtering.The latter approach simplifies filtering in that uniformly spacedsamples are always used. In some instances, this robust filteringapproach could provide better results than the median filtering and inmany instances, performs similarly as the median filtering-basedapproach.

In other embodiments, an alternative approach is employed for detectingsamples with strong noise. The received signal r_(k)(i) is multiplied bythe complex conjugate of its estimated modulation symbol and theresulting sample can be written as:y _(k)(i)=ŝ _(k)*(i)r _(k)(i)=h _(k)(i)+ŝ _(k)*(i)n _(k)(i)=h _(k)(i)+ñ_(k)(i)  (24)where ŝ_(k)*(i) represents an estimate of the i-th modulation symbol onthe k-th subcarrier, and it is also assumed, for simplicity, thatmodulation symbols have a unity power. Since h_(k)(i)≅h_(k)(i−1), onecan calculateΔy _(k)(i)=y _(k)(i)−y _(k)(i−1),i=2, . . . =h _(k)(i)−h _(k)(i−1)+ñ_(k)(i)−ñ _(k)(i−1)=ñ _(k)(i)−ñ _(k)(i−1)  (25)

If one noise sample is much stronger than the adjacent one, which is areasonable assumption in truly impulsive noise environment, Δy_(k)(i)will be large in absolute value. Then by comparing Δy_(k)(i) withΔy_(k)(i−1) and Δy_(k)(i+1), the index of strong noise sample could bedetermined. Similar analysis will help to identify two consecutivesamples that are large, which is a less likely event, as long as the twonoise samples are not such to approximately cancel each other, e.g., thesame amplitude but opposite phase. Once the strong noise samples areidentified as described above, robust filtering could be appliedaccording to one of the previous robust filtering embodiments.

The main difficulty in mobile communication systems operating over awide range of mobile speeds is to select a proper filter length, sincethe mobile device's speed and correspondingly the channel selectivityare not known in advance. Typically, the filter length is predeterminedto cope with fast channel variations, which in turn results insuboptimum performance at lower rates of channel dynamics. In oneembodiment, the level channel selectivity in time could be readilyestablished by measuring the mobile device speed, e.g., in receiverswith GPS capability. However, GPS may not be available in allsituations, and receivers—and other techniques—must also be provided forestimating the time selectivity of the channel. In one embodiment, thelevel crossing rate (LCR)-based estimator is used to estimate theselectivity of the channel. As LCR values represent channel dynamics,and as shown in FIG. 12, one example relates the optimum filter lengthsto channel dynamics. As such, look-up tables (LUTs) are constructed todefine the relationship (and thus mapping) between the optimum filterlengths and the time selectivity of the channel. An illustrative exampleof such an LUT is shown in Table 5. Table 5 shows conceptually mutualrelationship between the mobile device's speed, LCR and optimum filtertap length.

TABLE 5 LUT for LCR versus filter length N. Mobile device speed LCRFilter length V₁ LCR₁ N₁

V_(i) LCR_(i) N_(i) V_(i+1) LCR_(i+1) N_(i+1)

V_(n) LCR_(n) N_(n)

Since the time selectivity is not known a priori, the channel responseis estimated using a conservative filter length appropriate for fastchannels and from so obtained channel response, the LCR is measured.There are multiple ways in which LCR could be measured. In oneembodiment, the average level of channel amplitude response is estimatedand the number of times that amplitude response crosses the averageamplitude level is used as LCR estimate. In another embodiment, tominimize the impact of minor variations around the mean amplitude level,two or more levels about the mean level could be used to count levelcrossings.

To illustrate the use of LUT, which in practice is predetermined bysimulations or by analytic techniques, consider a measured LCR equal toLCR_(m) such that LCR_(i)<LCR_(m)<LCR_(i+1). Then the optimum filterlength could be found asN _(m)=Interp[LCR _(m),(LCR _(i) ,N _(i)),(LCR _(i+1) ,N _(i+1))],where Interp is desired interpolation function, e.g., linearinterpolation for simplicity. Even though there is some noisiness in theestimation of LCR, this approach for selecting an appropriate filterlength is favorable to using one filter length for all channelselectivity conditions.

A specific example of LUT based on extensive simulations is shown inTable 6 below. These results were obtained in one exemplary embodimentby simulating an urban frequency selective multipath channel model atdifferent mobile device speed and measuring BER and FER for variousfilter lengths in an OFDM receiver employing convolutional decoding. Itis apparent that optimum filter lengths for different speeds could varyover a very broad range, i.e., over one order of magnitude, signifyingthe importance of proper filter length selection.

TABLE 6 LUT for mobile device speed and LCR versus the filter lengthsfor initial and iterative channel estimation. Mobile Level Time domainfilter Time domain filter device speed Crossing tap length for initialtap length for iterative (km/h) Rate channel estimation channelestimation 2 1 FIR_tap = 111 FIR_tap = 81 4 2 FIR_tap = 81  FIR_tap = 6115 6 FIR_tap = 45  FIR_tap = 33 30 12 FIR_tap = 21  FIR_tap = 17 60 17FIR_tap = 15  FIR_tap = 15 150 20 FIR_tap = 3  FIR_tap = 3  >150 >20FIR_tap = 3  FIR_tap = 3 

The reliability of LCR estimation may be increased in multicarriersystems. More reliable LCR could be estimated by averaging LCRsestimated over multiple pilot/reference subcarriers at differentfrequencies. In this case, the average LCR helps to estimate the taplength more accurately. In one embodiment, the average LCR is measuredover all reference subcarriers. In another embodiment, M>=1 subcarrierswith best SNR are selected, based also on the noise power measurement,to calculate LCR.

Once the optimum filter length is selected based on LUT, the channelresponse estimation is repeated using the optimum estimated filterlength, using one of the approaches described in previous embodiments.Next it is desired to accurately estimate the noise power using pilotsymbols.

The noise power estimates, together with phase and amplitude estimates,collectively referred to herein as CSI estimates, are used for variousreceiver signal processing functions, such as FEC decoding and diversitycombining. Also, in one aspect of the invention, noise and signal powerestimates are used to estimate SNR to facilitate adaptive datasymbol-based decision directed channel estimation (DDCE), as it will beexplained subsequently.

The instantaneous noise estimates can be found as{circumflex over (n)}(i)=(r(i)−{circumflex over (h)}(i)p(i),for i=1,2, .. . ,  (27)where ĥ_(l) is the channel estimate for the i-th symbol interval. Whilethe noise is complex, its real and imaginary parts ({circumflex over(n)}_(Re)(i)=Re({circumflex over (n)}(i)), {circumflex over(n)}_(Im)(i)=Im({circumflex over (n)}(i)) could be calculatedseparately. Since in most communication systems, the noise is additiveand white, the noise power estimation is done by simply averaging thenoise power over a large number of samples as follows;

$\begin{matrix}{{{{\hat{n}}_{Re}}^{2} = {\frac{1}{M}{\sum\limits_{i = 1}^{M}{{{\hat{n}}_{Re}(i)}}^{2}}}}{{{\hat{n}}_{Im}}^{2} = {\frac{1}{M}{\sum\limits_{i = 1}^{M}{{{\hat{n}}_{Im}(i)}}^{2}}}}} & (28)\end{matrix}$

In some practical communication systems where the interference isdominant, however, especially in cases of time-varying and impulsiveinterference, the noise may be non-white. A typical example of noiseplus time- and frequency-selective interference is shown in FIG. 13. Itis apparent from FIG. 13, showing exemplary noise power variations intime and frequency domains, that for estimation of noise power level,averaging noise power over a relatively long period is not appropriate.Rather, the noise power estimates should be obtained using shorterfilter lengths, depending on the degree of noise power variations overtime and frequency domains. The real and imaginary noise power estimatescan be expressed as

$\begin{matrix}{{{{{\hat{n}}_{Re}(i)}}^{2} = {\frac{1}{L_{n}}{\sum\limits_{l = {i - U_{n}}}^{i + U_{n}}{{{\hat{n}}_{Re}(l)}}^{2}}}}{{{{\hat{n}}_{Im}(i)}}^{2} = {\frac{1}{L_{n}}{\sum\limits_{l = {i - U_{n}}}^{i + U_{n}}{{{\hat{n}}_{Im}(l)}}^{2}}}}} & (29)\end{matrix}$where U_(n)=(L_(n)−1)/2 and L_(n) is the tap length for noise powerestimation filter. Similarly as in channel estimation, suitable taplengths for the noise power estimation for different channel modelsbased on the LCR should be prepared in a separate LUT. In multicarriersystems, the noise power estimates for data subcarriers can be found byusing interpolation, e.g., linear interpolation, over the frequencydomain for both impulsive and non-impulsive noise cases. An exemplaryLUT for mobile device speed and LCR versus filter length for noise powerestimation is given in Table 7. The communication scenario for theresults in Table 7 corresponds to an FM HD Radio system in the presenceof host and first adjacent FM interference.

TABLE 7 LUT for LCR versus filter length for noise power estimation.Mobile device speed Time Domain filter tap length (km/h) Level CrossingRate for noise power estimation 2 1 FIR_tap = 19 4 2 FIR_tap = 19 15 6FIR_tap = 17 30 12 FIR_tap = 17 60 17 FIR_tap = 13 150 20 FIR_tap =5  >150 >20 FIR_tap = 3 

In many prior art systems, the CSI estimates for the data symbols areobtained based on interpolation of the pilot-based CSI estimates. Thatapproach is appropriate if the channel variations could be trackedeasily, such as in slow fading channels and high SNR conditions. For thecases where the channel has more selectivity, or the total number ofpilot symbols is typically insufficient, in some prior art systems thedecision-directed channel estimation with data symbols was also used.However, the decision-directed channel estimation performs poorly in lowSNR regime.

In one aspect of the invention, adaptive decision-directed channelestimation with data symbols is used to enhance the performance of thepilot-based channel estimation. Moreover, hybrid channel estimation isused to utilize channel state information obtained from both the pilotsymbols and a fraction of unknown data symbols. The adaptivecharacteristic of certain aspects of the invention is in that the numberof data symbols selected for decision-directed channel estimation mayvary from frame to frame as a function of the channel. For example, thenumber of data symbols may depend on channel selectivity or dynamics.More specifically, the number of symbols may be selected based on therapidity, speed or rate of variation of the channel. As another example,the number of symbols may be based on the noise realization, as it willbe explained in more detail below. That is, the decision-directed methodutilizes only data symbols with SNR higher than a predeterminedthreshold. Thus, in one embodiment, only a fraction of data symbols,more reliable ones, in the received symbol sequence, is used for theDDCE. A desired fraction of the decision-directed symbols depends on theselectivity of the communication channel as well as the noise level atthe receiver. One objective is to minimize the use of “bad” data symbolswith weak SNR that could cause error propagation in CSI estimation, thatis, to use less data symbols. Another objective is to use more datasymbols to improve CSI estimation in the presence of noise, especiallyin more selective channels. As these two objectives are contradictory,obviously there is a tradeoff which varies with channel selectivity,i.e., dynamics of changes of channel response.

In slower channels with longer channel estimator filter lengths, betterCSI interpolation is possible and one could use less data symbols forDDCE such to minimize the use of data symbols with low SNR. On the otherhand, in faster channels to follow channel variations better, shorterfilter lengths are desirable and more data symbols are preferable touse. In the latter case, using more symbols with lower SNRs may bebeneficial even though some of them may be erroneous, and the use ofmore data symbols will provide better CSI estimates. These principleswill be better understood by considering an exemplary embodimentdiscussed below.

Certain aspects of some embodiments may be applied to various MCsystems. For the example of an MC system with staggered pilot symbols asshown in FIG. 10b , it is instructive to consider examples of channelresponse variations in time and frequency domain illustrated in FIG. 14.More specifically, FIG. 14 illustrates impact of thresholds on themixture of pilot and decision-directed channel estimation. In slowfading channels depicted in FIG. 14a , channel response variation intime is slow such that over a relatively long period of time (i.e.,Pilot+DDCE period, 2176), the channel gain |H(f)| does not changerapidly. Consequently, in the region 2176 where the channel gain isabove a predetermined threshold, e.g., Thr₂, pilot plus DDCE channelestimation can be performed in higher accuracy. In the region 2177 wherethe channel gain is below the threshold, e.g., Thr₂, interpolation basedon pilot-based channel estimation would work well because of slowchannel variations. The use of lower threshold, Thr₁, would notnecessarily improve interpolation performance because of fairly gradualchannel changes, but would risk using erroneous data symbols, 2178, inDDCE, potentially compromising the accuracy of channel estimation. Thus,there is less need to use DDCE in the region where the channel gain issmall, and where data symbols are more prone to transmission errors.

Consider next a fast fading scenario as illustrated in FIG. 14b . If ahigher threshold, Thr₂, is used in the fast fading scenario, most of thetime the channel estimation will be based on pilots, 2179, andinterpolation between pilot symbol positions. However, due to fastchannel variations, interpolation may not be sufficiently accurate totrack rapid channel variations. In this case, it is preferable to use alower threshold such as Thr₁ to better follow channel changes byemploying pilot plus DDCE over longer time periods, 2180 and 2181. Thus,even though some data symbol decisions in DDCE may be erroneous byemploying Thr₁, this would be more than compensated by the use of moredata symbols in DDCE to track channel dynamics. With a lower threshold,the region where channel estimation is based solely on pilots andinterpolation, 2182, is correspondingly smaller with less impact onoverall performance, even though interpolation in that region may not besufficiently accurate. Correspondingly, a lower threshold for DDCE ispreferred. Thus, in general, in slow channels it is preferable to use ahigher threshold and less but more accurate data symbols in DDCE; infaster channels, it is preferable to use a lower threshold and more butless accurate data symbols in DDCE.

To summarize, in time periods where the channel gain is above aspecified threshold for DDCE, 2180 and 2181 in the example of FIG. 14b ,both pilot symbols, 2179, and data symbols, 2178, are employed forchannel estimation as described in other embodiments. In regions wherethe channel gain is below a specified threshold for DDCE, 2182 in theexample of FIG. 14b , channel estimation is performed using pilotsymbols with low pass filtering with an appropriate filter lengthcorresponding to channel dynamics, as described in other embodiments.Then, for data symbol positions in time period 2182, the channel isestimated by interpolation techniques as known in the art, e.g., linearor polynomial interpolation, using pilot-based channel estimates in thecorresponding region and channel estimates from adjacent segments, 2180and 2181, where both pilot-based estimation and DDCE were employed.After such channel estimation is performed, LCR could be estimatedaccording to some of the embodiments previously described. Noise powerestimation is performed similarly as channel estimation; in some timeperiods, based on both pilot and data symbols where the channel gain isabove a threshold, and based on pilots only and interpolation where thechannel gain is below a threshold.

Referring again to an MC system with staggered pilot symbols asillustrated in FIG. 14b , once the channel estimation on referencesubcarriers carrying pilot symbols is performed in time domain, it isdesired to estimate channel on data subcarriers carrying only datasymbols. Consider a slow fading channel scenario illustrated in FIG. 14c. Channel estimates based on time domain channel estimation areavailable for pilot subcarriers 2189. Again for subcarriers in thefrequency region 2183 in which the channel gain on pilot symbols isabove the threshold, the channel estimation would be based on both pilotand data symbols, using DDCE, while in the region 2184, the channelresponse on data subcarriers positions would be obtained by performinginterpolation using pilot position channel estimates, similarly asexplained for the time domain case. A similar approach is applied in thefast fading case corresponding to FIG. 14d , where it is assumed that ata specific point in time, the channel frequency response is the same forfast and slow (in time) fading channel. The difference is that accordingto the threshold selection explained earlier based on LCR consideration,the threshold for the fast fading case is lower and, thus, more datasubcarriers will be used in DDCE and interpolation will be usedpreferably in relatively small region of frequencies. Thus, overall,taking into account both time and frequency domains, in fast fadingchannels the channel estimator will have more data symbols forestimation of the channel response than in slow fading channels. Eventhough this may result in a higher probability of erroneous symbols usedfor channel estimation in fast fading case than in slow fading case,this will enable better tracking of channel selectivity in the fastfading case, which is more critical in fast fading than in slow fadingcase. Thus, more symbols used in fast fading for tracking channeldynamics will more than compensate for the possible presence of moreerroneous symbols due to the lower threshold. In one embodiment, theestimated time selectivity using LCR is used to facilitate not only theoptimal selection of filter coefficients but also the SNR threshold forselecting a subset of data symbols for DDCE. That is, for a given levelof time selectivity of the channel, all symbols with estimated SNRlarger than a specified threshold corresponding to that level ofselectivity, are used for DDCE. On symbol positions where estimated SNRis smaller than the respective threshold, interpolation is used based onpilot symbols and possibly DDCE estimates in adjacent segments withbetter SNRs. This alternative embodiment could be used when noise plusinterference power exhibits time and frequency variations so thatchannel response gain alone is not sufficient to decide which symbolscould be used for DDCE. The exemplary embodiment used to obtain Tables 6and 7 was also used to determine optimum thresholds for DDCE for variousmobile device speeds, such to minimize BER and FER of the exemplarysystem under consideration. The corresponding results obtained byextensive simulations are summarized in Table 8. By inspection, it couldbe seen that as the mobile device speed and correspondingly channel LCRincrease, a smaller threshold is preferable, meaning that at highermobile's speeds, more data symbols would be used for DDCE, consistentwith previous explanations of certain aspects of the invention.

TABLE 8 LUT for LCR versus DDCE Threshold for different channel models.Mobile device Threshold for the Decision speed (km/h) Level CrossingRate Directed mode 2 1 Threshold = 0.75 4 2 Threshold = 0.65 15 6Threshold = 0.6  30 12 Threshold = 0.5  60 17 Threshold = 0.25 150 20Threshold = 0.2  >150 >20 Threshold = 0.2 

In multicarrier systems, the optimum filter length for smoothing infrequency domain could be selected based on the level of frequencyselectivity in the channel response, similarly to how it was done withLCR estimation to match the time selectivity of the channel. Tofacilitate that, in one embodiment, an LUT is constructed, e.g., basedon simulations, measurement, analytical or other techniques thatestablishes the relationship between frequency selectivity of thechannel and optimum filter length for filtering in the frequency domain.

In one aspect of the invention, the frequency domain smoothing withoptimally selected filter length is applied to the channel and noisepower estimation after time domain processing. There are multiple waysin which the frequency selectivity of the channel could be estimated. Inone exemplary embodiment, the frequency selectivity is estimated bymeasuring the rate of change of channel variations over a certain rangeof frequencies. If the measured channel changes were more pronouncedover the range of frequencies Δf, the channel frequency response islikely more selective and correspondingly shorter filter lengths forsmoothing in frequency domain should be used, and vice versa.

To construct an LUT for optimum filter lengths versus frequencyselectivity, several channel models with varying levels of frequencyselectivity from low to high, such as urban, sub-urban, rural, etc., areused in the simulator of HD Radio OFDM system to analyze the impact offrequency selectivity on the optimum filter length for the frequencydomain processing. Alternatively, the LUT could be obtained byanalytical techniques or from measurements, or by using otherapproaches. Intermediate values could be obtained by interpolation, asexplained earlier. In cases when noise and/or interference may benon-white over the frequency domain, it is necessary to establish an LUTfor filter length selection for noise power estimation, i.e., smoothingover frequency domain, similarly as described earlier for other cases.The frequency selectivity could be estimated as a measure of the channelgain change over a range of frequencies corresponding to K subcarriers,measured at multiple OFDM symbol intervals and represents the averagevalue of Q largest measured channel gain changes. This averaging isapplied to minimize the impact of noise. As could be seen from Table 9,higher frequency selectivity implies a shorter filter length, and viceversa. It is important to note that the filter length may vary by asmuch as a factor of 2.4 between most and least selective channels amongconsidered channel scenarios. The LUT for the analyzed exemplaryembodiment is given in Table 9.

TABLE 9 LUT for frequency selectivity parameter delta versus filter taplength for different frequency selective channel models. Channel ModelDelta Tap length Less Frequency Selective Channel Δ ≦ 0.001 FIR_tap = 65Less-Moderate Frequency Selective 0.001 ≦ Δ ≦ 0.1 FIR _tap = 35 ChannelModerate Frequency Selective 0.1 ≦ Δ ≦ 0.2 FIR_tap = 31 Moderate-VeryFrequency Selective 0.2 ≦ Δ ≦ 0.3 FIR_tap = 27 Very Frequency SelectiveΔ ≧ 0.3  FIR_tap = 23

In some embodiments, aspects of previous embodiments are integrated toprovide the initial advanced CSI estimation that can substantiallyenhance the performance of receivers. Such an approach could be used inreceivers without iterative decoding as well as an initial stage inreceivers with iterative CSI estimation and FEC decoding. FIG. 15illustrates an exemplary processing flow for such an advanced CSIestimation.

The input to the Advanced CSI Estimation 2190 (and thus to the overallprocessing flow), line 2191 represents the received signal containingpilot symbols and data symbols (also referred to as the “receivedcomposite signal” for brevity), a pilot-symbol portion of the receivedcomposite signal (referred to as the “received pilot signal” forbrevity) carrying the complex pilot symbols transmitted on referencesubcarriers.

CSI estimator in block 2192 produces channel estimates using pilotsymbols and employing a filter tap length corresponding to fastchannels, as described in the previous embodiments. In this exemplaryembodiment, a multicarrier system with dedicated pilot subcarriers isconsidered.

Then, the CSI estimates from block 2192 are further processed in theblock 2193 where the time selectivity of the channel is estimated byestimating the level crossing rate (LCR) and filter tap lengths, basedon an LUT, in accordance with embodiments of the invention.

The updated filter tap lengths from the block 2193 are used in block2194 to obtain improved channel estimates with an optimized filterlength. Similarly, noise power estimation is also performed by using theoptimized filter tap length selected from an LUT for a given level oftime selectivity.

The channel and noise power estimates from block 2194 for the pilotsubcarriers are then low-pass interpolated in frequency domain in block2195 to produce interpolated channel response and noise power estimatescorresponding to the data subcarriers in the symbol sequence.

Then, the resulting CSI estimates for the data signal are furtherrefined in the Adaptive Decision Directed (ADD) approach block 2196, inaccordance with the embodiments where selected reliable symbols, withSNR above a Threshold that depends on the time selectivity of thechannel, are used to estimate channel and noise power.

In block 2197 the frequency selectivity of the channel response isestimated and correspondingly a filter tap length is selected from anLUT, in accordance with previously described embodiments.

Additional frequency domain smoothing using both pilot and data symbolsis performed in block 2198 using the optimum filter tap length estimatedin block 2197. Interpolation is employed for symbols with SNR below theThreshold, where DDCEs are not available.

Finally, such refined CSI estimates are output to line 2199 to be usedin the subsequent decoding stage of the receiver, as it will beexplained below in more detail.

Decision directed channel estimation in the initial step provideslimited gains because only a subset of data symbols is used and stillsome of data symbols used for DDCE may be erroneous and correspondinglycorrupt CSI estimation. In one embodiment, corresponding to FIG. 16, CSIestimation and FEC decoding are performed iteratively. Generallyspeaking, after the advanced initial CSI estimation following theprocedure 2190 in FIG. 15, FEC/SISO decoding 2215 is performed. Afterthe FEC decoding, better estimates of coded symbols are available andused for another iteration of advanced CSI estimation. Morespecifically, in FIG. 16, block 2212 performs initial CSI estimation asexplained in the embodiment corresponding to FIG. 15 and other CSIestimation embodiments. Received noisy channel symbols and CSI estimatesproduced by block 2212 are processed in de-mapping block 2213, toproduce coded bit LLRs, as it is known in the art or as described inother embodiments. Coded bit LLRs are, optionally, de-interleaved inblock 2214 if interleaving was employed in the transmitter.

A SISO decoder 2215 performs FEC decoding in accordance with variousembodiments, depending on which FEC code might have been employed. TheSISO decoder may produce two types of outputs, 1) LLRs of informationbits for making final decisions or for processing in subsequent stagesof the receiver, or 2) coded bit LLRs on line 2216. The latter is ofinterest for iterative CSI estimation. In the feedback loop foriteration, the coded bit LLRs are optionally interleaved, if aninterleaver was employed in the transmitter, and used to form channelsymbols in block 2218 that are fed back to Advances CSI Estimator 2212for next CSI estimation iteration.

Before the de-mapping block 2213, additional erasure detection may beperformed based on the identification of samples with very large noiserealizations, as it may happen in scenarios with impulsive noise and/orinterference. In this case, the indexes of the erasure positions, withlarge noise samples, are also supplied to put zeros instead ofcalculated LLRs for the corresponding composite signal samples.Otherwise, LLR calculation is performed for each and every receivedsymbol. It should be noted, that the described erasure approach may alsobe implied in the first stage of CSI estimation and LLR calculations,before first FEC decoding.

The estimates of coded bits (i.e., LLRs of coded bits on line 2216)obtained from FEC decoding 2215 are used to produces estimates ofchannel symbols that are more reliable than decision directed symbolscalculated at the initial step (in block 2215 before the iterativeprocessing). The output from the SISO decoder 2215, is fed back to themapping block 2218 via an interleaving block 2217. Mapper 2218 maps LLRestimates of coded bits after FEC decoding to channel symbols.Consequently, the output from 2218 provides more reliable estimates ofthe transmitted symbol sequence that it is used for the next iterationof CSI estimation. In one embodiment, the reconstructed channel symbolsare “hard” symbols; that is, binary coded bits are mapped to modulationconstellation as in the transmitter. In another embodiment, “soft”symbols are employed, which account for the reliability of the codedbits estimated during the FEC/SISO decoding in block 2215.

“Soft” symbols for BPSK signaling could be constructed, with symbolss_(k)(i)ε{+v, −v}, as:

_(k)(i)=v tan h(LLR(s _(k)(i))/2)  (30)where LLR(s_(k)(i)) denotes the LLR for bit

_(k)(i), and is calculated from the received signal as known in the artand explained in other embodiments. In the case of QPSK modulation, softbits for in-phase (I) and quadrature (Q) channels,

_(k,I)(i) and

_(k,Q)(i), respectively, could be calculated from the corresponding LLRsas shown above and soft, complex, QPSK symbols could be constructed as

_(k)(i)=

_(k,I)(i)+j·

_(k,Q)(i).

Also, in each subsequent iteration, channel and noise power estimationis done by the soft symbols on line 2219. The principal differencerelative to the embodiment describing the advanced initial CSIestimation is that there is no adaptive DDCE in subsequent iterationswhen estimated “soft” symbols are employed. That is, all regenerateddata symbols on line 2219, along with pilot symbols in the symbolsequence, are used for CSI estimation. If “hard” symbols are used, thenthe least reliable ones could be skipped in CSI estimation, as before.Additional median and smoothing filters are also applied in block 2212similarly to the first iteration. Estimation of the time- andfrequency-selectivity may also be updated in this block with morereliable symbols used in the iteration. The same steps are followed forprocessing after block 2212 in the second iteration onward.

Another principal difference relative to the initial advanced CSIestimation is that somewhat shorter filter lengths are found to beoptimal in subsequent iterations, since a larger number of estimateddata symbols of sufficient accuracy are available for CSI estimation.These shorter filter lengths for subsequent iterations are illustratedin

Table 6 earlier. A desired number of iterations for CSI estimation andFEC decoding could be performed. In prior art methods, often severaliterations for CSI estimation and FEC decoding are performed. In certainembodiments, one additional iteration after the initial processing,i.e., the initial forward-path processing from block 2212 through block2215, may suffice. This is enabled by the advanced initial CSIestimation and optimization of CSI estimation parameters in thecorresponding steps, based on channel selectivity according to certainaspects of the invention described in various embodiments. Thus, theiterative CSI estimation employing certain embodiments achieves asignificant reduction in overall computational complexity and asubstantial gain in performance compared to other prior art methods ofiterative CSI estimation. For the non-iterative CSI estimation, however,the performance improvement in CSI estimation would come at the expenseof computational complexity somewhat higher than in prior art methods.

CSI Estimation for Diversity Combining Systems

Maximal ratio combining (MRC), faces significant challenges in scenarioswhere one or more diversity channels experience low SNR conditions andmay result in mismatched combining. That is, due to noisy, erroneous CSIestimates, a bad channel may be given more weight than appropriate, andthus pollute the diversity-combined signal, resulting in a degradationof performance.

In one embodiment of the invention, the innovative aspects of CSIestimation for non-diversity systems described earlier are extended todiversity systems.

In the initial step of CSI estimation, with use of a shorter filterlength to estimate LCR, best diversity channel is used to estimate LCR.Using LCR-based selection of optimum filter-tap lengths in the timedomain, the pilot-based CSI estimation is performed to equal-gaincombine the diversity channels. Equal gain combining (EGO) essentiallymeans phase-coherent combining of different channels with equal weight.That is, for this step, only phase estimation is required. Individualdiversity channels and equal-gain combined SNRs are illustratively shownin FIG. 17 for a second-order diversity system. Depending on athreshold, Threshold1 to Threshold4, 2221, it can be seen that forindividual diversity channels 2224 and 2225, SNRs would be below thethreshold for a significant fraction of time, and corresponding signalsegments would not be useful for decision-directed CSI estimation. Onthe other hand, for a reasonable threshold selection, the equal gaincombined SNIR 2223 may most of the time be appropriate to make datasymbol decisions to facilitate decision-directed CSI estimation. Thus,on the selected set of symbols with SNR larger than the selectedthreshold, based on the equal-gain combined signal, data symbolsdecisions are made and used for complete CSI estimation. This isessentially similar to the adaptive DDCE approach mentioned earlier inthe non-diversity CSI estimation embodiments. Specifically, to obtainmore accurate phase, amplitude, and noise power estimates for each ofdiversity channels, both pilot symbols and selected set of data symbolsthat are more reliably decoded, after equal gain combining, are used.With such improved CSI estimates, MRC combining of diversity channels isthen performed as known in the art. All other aspects of CSI estimationremain substantively the same as in the non-diversity CSI embodimentsdescribed earlier.

In yet another embodiment for diversity combining, additionalimprovement is facilitated. Once complete CSI estimates are availablefor each individual channel as described in the previous embodiment, MRCand EGC are applied selectively. Define a threshold, Thr, for SNR suchthat for SNR< Thr, CSI estimation error is unacceptable and may degradeMRC performance.

The selective MRC and EGO approach is implemented as follows:

-   -   1. Identify all symbol regions/segments, S_(MRC), such that        SNRi> Thr, i=1, . . . , L,        -   where L is the number of diversity channels.    -   2. If a data symbol belongs to the set, S_(MRC), perform MRC,        -   else if data symbol does not belong to S_(MRC), perform EGO.

In another embodiment, the receiver is provided a feedback channel toreport optimal or near optimal structure of pilot signals based on thechannel selectivity in time, and possibly in frequency, and/or SNR. Inmodern communications systems, such as 3G/4G cellular and WiFi,modulation and FEC coding rate, and possibly MIMO parameters, areadaptively adjusted based on the received signal measurements at thereceiver. In some systems, adaptive power control may be employed inaddition. This considerably improves the link throughput by allowing ahigher throughput when the SNR is relatively high, and a lowerthroughput when the SNR is lower. However, this approach is stillsuboptimum in that the pilot structure is fixed and more pilot symbolsare preferable during high channel selectivity conditions and/or lowerSNR. For example, during times of higher channel selectivity, a higherdensity of pilots enables better CSI estimation to support higher ordermodulation schemes. This, in turn, enables the transmission of moredata, which more than compensates for increased pilot overhead.Alternatively, more pilot symbols for the same modulation order enablesreceiving data symbols more accurately. Thus, the throughput of moderncommunication systems may be improved by using an adaptive pilotstructure in addition to adaptive modulation and FEC coding rate andpossibly MIMO parameters, all of which could be facilitated bymeasurements of the received signal at the receiver and reporting anappropriate metric or metrics via a feedback channel to the transmitter.

In another embodiment, the pilot structure is optimized not just basedon channel selectivity and possibly signal strength, but also inconjunction with possible modulation and FEC coding scheme operatingpoints. For example, higher order modulation schemes and or less FECredundancy typically require more accurate CSI estimation and,correspondingly, more pilot symbols are desired. A combination ofmodulation type, FEC code rate and possibly MIMO parameters is oftenreferred to as modulation-coding scheme (MCS) index. In one embodiment,pilot structure could be selected or optimized based on channelselectivity for each possible MCS index. Each MCS index may be furthersubdivided into multiple MCS options based on channel selectivity, thusessentially creating a larger set of MCS indexes, where the transmissionscheme may be adjusted such that it is more in tune with channelvariations, thereby enabling performance improvement.

In an exemplary embodiment, four pilot structures could be employed, forlow, medium, fast and very fast mobile device speeds, corresponding toincreasing densities of pilot symbols. Thus, with two bits of additionalinformation feedback, the adaptive pilot structure could be implemented.In one exemplary embodiment, these two bits of pilot structure feedbackcould be transmitted in addition to channel quality indicators foradaptive modulation, FEC coding rate and MIMO parameters. In anotherembodiment, the pilot structure indicator bits could be combined withchannel quality indicators for other mentioned purposes, such that oneindicator could describe multiple attributes, including the pilotstructure, of the communication transmission. The pilot structurefeedback could be based on multiple measured received signal attributes.For example, it could be based on time selectivity of the channel, or incase of multicarrier systems, both time and frequency selectivity couldbe used to determine the preferable pilot structure. Selection ofpreferred pilot symbol structure or density could be facilitated by theuse of one or more look up-tables. Alternatively, one or more thresholdscould be used, or other techniques such as pre-stored performancecurves.

In addition, as mentioned earlier, the SNR could also be employed toprovide a more refined pilot structure selection. For example, at agiven SNR, a higher speed of the device may require a pilot structurewith more pilot symbols per frame/packet. However, when the average SNRper packet is higher, less pilot symbols may be needed in some range ofmobile device speeds. Thus, it is possible, for example, to use the samepilot structure for a lower and a higher device speed when the SNR ishigher during periods of higher speed then during periods of lowerspeed. Alternatively, instead of the SNR, other indicators of channelquality could be used, such as received signal power, received signalstrength indicator, received code power in CDMA systems, and/or BER orFER, as they are known in the art. It is apparent that the systemdesigner could use multiple measured received signal attributes toestimate optimum or near-optimum pilot structure, as well as to use adesired number of possible pilot structures to balance the complexityand the performance gains.

III. Applications to Concatenated Coding Systems

This subsection discusses systems and methods for combining thetechniques discussed in the previous two subsections, as well asadditional aspects, for advanced decoding of concatenated codingsystems. These aspects could be used in various communications systemsthat utilize concatenated coding schemes, examples of which arediscussed below.

A general system model for concatenated encoding and decoding is givenin FIG. 18. The outer FEC encoder 3101 encodes information bits 3114into coded bits 3115 and typically employs a linear block code such as aRS (RS) code or a BCH code in the standards discussed below, but otherouter codes may be used as well. An outer interleaver 3102 typicallyfollows the outer FEC encoder 3101 with a matching outer de-interleaver3110 in the receiver. The outer interleaver 3102 changes the order ofcoded bits 3115 to produce a different order of coded bits 3116,according to the interleaving algorithm. The outer interleaver andde-interleaver pair is used to disperse error bursts from the output ofinner FEC decoder 3109 in the receiver so as to enable the outer FECdecoder 3111 to decode correctly. The outer interleaver could be bit orbyte/symbol based. Examples of prior art systems that have adopted suchan outer FEC code and an outer interleaver include Digital VideoBroadcasting-Handheld (DVB-H), Digital Video Broadcasting-Terrestrial(DVB-T), Digital Audio Broadcasting (DAB), Terrestrial-DigitalMultimedia Broadcasting (T-DMB), World Space System, China MobileMultimedia Broadcasting (CMMB), Satellite Digital Radio (SDR) systems(US, Europe), and MediaFLO.

In the DVB-H standard, each MPEG-2 packet of 188 bytes is encoded usinga RS (204, 188) outer FEC code, shortened from RS (255, 239) code, whichis implemented as follows: 51 all-zero bytes are added at the beginningof the 188-byte packet to form a 239-byte block. This block is encodedusing a systematic RS (255, 239) code. After encoding, the first 51bytes are discarded and remaining 204 bytes are transmitted. The outerinterleaver is a byte-wise convolutional interleaver with depth I=12,implemented based on Forney approach. The interleaving is such that SYNCbytes are at space of 204 bytes from each other.

DVB-T for broadcast transmission of digital terrestrial televisionemploys a RS (204, 188) code as the outer FEC code. In DAB, for forwarderror correction and outer error protection, RS (204,188) and outerinterleaving can be applied to sub-channels carrying service componentsin packet mode in order to further increase the error robustness of DABdata delivery. In T-DMB, the outer FEC coder has a RS coder and a Forneyconvolutional interleaver. RS (204, 188) is used in T-DMB, which isobtained from RS (255, 239). In World Space System, the broadcastchannel is FEC coded by concatenating a RS (255, 223) block coder,followed by a block interleaver. CMMB uses a RS code as the outer FECcode along with a block byte outer interleaver.

In European Telecommunications Standards Institute (ETSI) SDR systems,the MPEG Transport Stream (MPEG-TS) is protected by an outer BCH code.Up to 8 MPEG-TS packets, each having a size of 188 bytes, aretransmitted at the same time. Error correction and detection isperformed by using one shortened BCH (3057, 3008) code for each 2MPEG-TS packets. The outer error correction code (overall minimumdistance dmin=10) is actually an outer BCH (3056, 3008, 9) code (withminimum distance dmin=9) concatenated by an inner single-parity checkcode (3057, 3056, 1). The BCH code is obtained by shortening anarrow-sense binary BCH (4095, 4047, 9) code.

The SDR in the U.S. (Sirius and XM satellite systems) operates onfrequencies between 2320 MHz and 2345 MHz. Sirius Satellite Radiooperates in the lower 12.5 MHz block of S-Band spectrum between 2320 MHzto 2332.5 MHz, and XM in the higher 2332.5 MHz to 2345 MHz portion. TheSirius band plan is divided between three possible signals: twosatellite signals employing single carrier transmission and oneterrestrial OFDM based repeater signal. In the urban areas, where theline of sight reception of the satellites is difficult or not possible,the service is covered by terrestrial repeaters adopting a multi-carriermodulation scheme, i.e., OFDM. The outer FEC code in Sirius SDR is a RS(255, 223) code.

In most of the systems mentioned above, inner FEC encoder 3103 is basedon convolutional codes but other codes such as turbo or LDPC codes orany trellis or block code may be employed. For example, a non-systematicIRA code, such as the one described in U.S. patent application Ser. No.14/607,043 entitled Systems and Methods for Encoding and Decoding ofCheck-Irregular Non-Systematic IRA Codes (herein referred to asinventions pertaining to check-irregular non-systematic IRA codes), thecontents of which are fully incorporated by reference herein in theirentirety, could be employed as an inner FEC code. Interleaved coded bits3116 are encoded by inner FEC encoder 3103 into a sequence of inner FECcode bits 3117.

An example of a concatenated system that relies on an inner turbo codeis Qualcomm's MediaFLO. Forward Link Only (FLO) Air Interface is a keycomponent of the MediaFLO system developed by Qualcomm as an alternativemobile multicast technology for the efficient transmission of multiplemulti-media streams to mobile devices using TV and multi-media channelbandwidths in VHF, UHF, or L-band. The FLO physical layer uses OFDM asthe transmission technique. In FLO Air Interfaces, a turbo inner codewith code rate ⅕ is used for transmitting critical overhead information,and rates {⅓, ½, ⅔} are for transmitting Multicast Logical Channels. Thehigher code rates are obtained from the base code rate using puncturing.A RS erasure correcting code is used as the outer FEC code. It is a RS(N,K) over the Galois Field with 256 elements, GF (256), with N=16 and Kchosen from the set {8, 12, 14, 16}. The case of K=16 corresponds to thecase when no RS encoding is actually performed. As another example, CMMBuses an LDPC code (rates ½, ¾) as the inner FEC code while using a RScode as the outer FEC code.

Optional inner interleaver 3104 and inner de-interleaver 3108 aretypically employed to break up and disperse error bursts at the outputof the channel, which typically occur in correlated fading conditions.Usually convolutional or block bit interleavers are employed, but incase of LDCP codes, an inner interleaver may not be required although insome instances may still be useful, e.g., with bit interleaved codedmodulation. Inner FEC code bits 3117 are interleaved in 3104 to produceinterleaved sequence of bits 3118 that is further passed to the symbolmapper 3105.

Some examples of the use of inner convolutional coding are providedbelow. In DVB-H, the inner FEC code consists of a mother 64-state rate ½convolutional code. Different puncturing patterns are designated toallow for achieving different code rates such as ½ (no puncturing), ⅔,¾, ⅚, 7/8. The transmitter can operate in non-hierarchical andhierarchical modes. In hierarchical mode, data includes High-Priority(HP) and Low-Priority (LP) parts. In case of hierarchical transmission,the transmitter has the ability to assign a lower code rate (moreredundancy) for HP data and a higher code rate for LP data. DVB-T forbroadcast transmission of digital terrestrial television employs apunctured convolutional code with five valid coding rates ½, ⅔, ¾, ⅚,and ⅞ as the inner FEC code. In DAB, the channel coding is based on aconvolutional code with constraint length 7. Different puncturingpatterns are designated to allow for achieving different code rates. InT-DMB, convolutional coding with puncturing is used as the inner FECcode. In WorldSpace System, a Rate ½ convolutional coder is used. In SDRsystems, the inner FEC code is a turbo code employing two systematic,recursive, convolutional encoders connected in parallel, with a turbointerleaver, preceding the second convolutional encoder. The outputs ofthe convolutional encoders are punctured to achieve different coderates.

In SDR in the US, the satellite and the terrestrial signals carry thesame Payload Channels (PC), included in a 432 msec framed packet with RSprotection (outer FEC encoding). The content of both Sirius satellitesis FEC coded with multiple channel coding schemes, includingconcatenated RS-convolutional coding and convolutional channelinterleaving. A punctured rate ⅜ convolutional inner FEC encoder (from amother code of rate ⅓) is used. Each satellite transports one half ofthe non-punctured and interleaved coded bits resulting in an effectiveinner encoder rate of ¾, such that two rate ¾ codes are complementary.An enhanced modulation format (hierarchical modulation coupled withadvanced channel coding) has been added to increase the total throughputof the system to 5.4 Mbps. The terrestrial bit-stream is a repeatersignal carrying same content as the satellite signals, but withdifferent FEC encoding and modulation. The inner FEC encoder forterrestrial repeaters employs a punctured rate convolutional code ofrate ⅗ (from a mother code of rate ⅓). Eventually soft bits from thesatellite and terrestrial receiver chains could be diversity combinedfor improved performance in areas of overlapped terrestrial andsatellite coverage.

Mapping of coded bits from interleaver 3118 to modulation symbols 3119is performed in 3105 with methods well known in the art, such as BPSK,QPSK, various forms of M-QAM, M-PSK or combinations of ASK and PSK, etc.Transmission could be of a single-carrier type or multi-carrier typesuch as OFDM, which may be system-specific. Furthermore, some form ofspread spectrum could be employed as well. Such details are omitted hereas persons of ordinary skill in the art would know how to use suchtechniques in conjunction with the aspects and embodiments describedherein. Various other examples of communication systems such as wirelesslocal area networks, cellular networks, infrared, acoustic, wireline andfiber optic systems could also be represented with the generalconcatenated structure described above.

Decoding of Concatenated Codes

Outer FEC decoding could be implemented with known methods in the art,but is preferably performed by utilizing aspects of the inventiondescribed in previous embodiments. Specifically, the outer FEC codescould typically be represented with binary parity check matrices anddecoded using BP based decoding algorithms, as described in previousembodiments discussed above in the first subsection pertaining to BPbased decoding of H/M/LDPC codes. Other than great performance withaffordable complexity, another advantage of BP based decoding methods isthat the soft decoder output could be used to derive extrinsicinformation to be fed back to a soft a priori input of the inner FECdecoder to facilitate iterative decoding between inner and outer FECdecoders. In case the BP based decoding of the outer FEC code convergesto a codeword, the output LLRs could be enhanced, i.e., quantized tohave a large absolute value that's configurable. This is furtherjustified by confirmation that decoded code word/s are verified as errorfree by error detection decoder, such as a cyclic redundancy check (CRC)decoder, for example. Since the probability of an undetected codeworderror would be orders of magnitude smaller than probability of bitdecoding errors, enhanced LLRs could be used as extrinsic informationi.e., a priori information for the inner FEC decoder. Thus, the enhancedLLRs obtained from such codewords are used as soft a priori inputs tothe inner FEC decoder. One embodiment of the invention is directed tospecific realization of BP-based decoding algorithms and iterativedecoding with soft reliability values as well as incorporation ofenhanced LLRs based extrinsic information in the iterative decodingprocess.

More specifically, the receiver block diagram in accordance with certainembodiments of the invention is depicted in FIG. 18. At the receiver,initial channel state information (CSI) estimation is performed 3106 onreceived, noisy modulation symbols 3120, followed by symbol de-mapping3107 that produces (coded) bit LLRs 3122 using the output of the CSIestimation 3121 and noisy modulation symbols 3120. For initial CSIestimation, conventional methods could be employed. Alternatively, theaspects of the invention described in the second subsection abovediscussing various embodiments for CSI estimation may be used in orderto improve the overall performance of the receiver. Then innerde-interleaving is performed in 3108 on the (coded) bit LLRs 3122. Theoutput of the inner de-interleaver 3123 is then decoded by the inner FECdecoder 3109. In some embodiments the inner interleaver/de-interleavermay be omitted, as discussed earlier. In some embodiments, the inner FECdecoding produces a sequence of soft bit outputs, say LLRs. For examplethis could be accomplished by means of a Log-MAP decoder or BP decoder,as typical examples. The sequence of soft bit outputs 3124, say LLRs,from the inner FEC decoder is de-interleaved in outer de-interleaver3110 and de-interleaved output 3125 is fed to the outer FEC decoder3111. The de-interleaving in 3110 could be bit or symbol/byte based, orcould be omitted, depending on the nature and the structure of the outerFEC code. The estimated transmitted information is extracted from theoutput of the outer FEC decoder at line 3130. The above describedprocess represents a simple sequential decoding as in conventional,non-iterative decoding in the art, or just first decoding iteration initerative decoding of concatenated codes.

In other embodiments, in addition to a sequence of soft bit outputs, theinner FEC decoder could also produce a list of hard decision sequencesamong which transmitted sequence could be found with a high likelihood.For example, in cases where the inner FEC code is a convolutional codeor a turbo code, a list Log-MAP decoder can be used to generate a set ofsoft bit outputs, e.g., LLRs, along with a list of M hard-decisionsequences ordered based on their likelihood of being correct. That is,the first hard decision sequence on the list is the most likely one, thesecond hard decision sequence on the list is the second most likely,etc. These sequences are de-interleaved, if interleaving/de-interleavingis employed, along with the soft bit outputs. Before performing softouter FEC decoding in block 3111 using the de-interleaved LLRs on line3125, first the de-interleaved hard decision sequences in the list arechecked to see if any of them corresponds to a valid codeword of theouter code. If so, there will be no need to perform soft outer FECdecoding. In some embodiments, if the inner FEC decoder/decoders did notproduce a valid codeword/s, outer soft decision decoding is performed.If after soft outer FEC decoding a valid codeword is not produced, softextrinsic information 3126 is interleaved in 3112 and fed back 3127 tothe inner FEC decoder 3109, to facilitate iterative decoding. In otherembodiments, even if none of the sequences corresponds to a valid outercodeword, simple hard decision error or error and erasure decoding canbe applied in 3111, using known such algorithm in the art (for exampleBerlekamp-Massey (BM) algorithm as described in the previousembodiments) on some (highly ranked on the list) or all sequences on thelist and only if it fails, soft channel decoding is performed. Again, ifafter soft outer FEC decoding a valid codeword is not produced, softextrinsic information is fed back to the inner decoder to facilitateiterative decoding, as explained earlier. The steps of performing listdecoding and/or hard decision error or error and erasure decoding beforeouter code soft decoding helps reduce the overall complexity of theouter code decoding and improve performance. The complexity is reducedbecause steps of list decoding and hard decision error and erasuredecoding could correct most of the error patterns, and since theircomplexity is typically much smaller than that of soft outer FECdecoding, the overall decoding complexity is reduced. The performancegain results from observations that some error patterns may be correctedby above steps while they may not be correctable by the soft outer FECdecoder. Specific details how list decoding could be performed inconjunction with the outer FEC code depends on, for example, how outercode codewords relate to inner code codewords, possibly viade-interleaving. More details on this aspect of the invention areprovided in the examples discussed in the fourth subsection belowpertaining to advanced HD Radio decoding. Specifically, it will bedescribed how list decoding is performed when one outer code codewordcorresponds to one inner code codeword (without outerinterleaving/de-interleaving), as in the case of MPS PDU header decodingin P1 channel, and also how list decoding is performed if one outer codecodeword includes bits/symbols from two or more inner code codewords,coupled via outer byte interleaving/de-interleaving, as in the case ofAAS data decoding in P3 channel. Implementations for other possiblevariations should be apparent to those skilled in the art, from thosetwo provided examples.

In many systems, before outer FEC encoder 3101, or before FEC encodingin general if an outer code is not employed, there is some kind of errordetection code encoding. Typically a CRC code is employed for errordetection. In that case, there could be a CRC decoder after the outerdecoder 3111. Using this CRC decoder, the validity of the outer decodedcodeword is checked. As soon as a codeword passes CRC check, it isaccepted. The CRC code can also facilitate list Log-MAP decoding whenthe outer code is systematic, otherwise the outer code could be used forerror detection to facilitate list decoding, as these approaches areknown in the art. In cases when a CRC code is employed, a valid outercodeword is accepted if it passes the CRC check.

In order to improve the performance of the receiver, as mentionedearlier, in some embodiments iterative decoding is performed as shownwith dashed lines in FIG. 18. A soft output decoder 3109 can be used fordecoding the inner FEC code which will provide soft inputs 3125, afterde-interleaving of soft outputs 3124 in 3110, to the outer FEC decoder3111. As explained before, any outer code that can be represented by abinary parity check matrix can be decoded by BP-based softchannel-decoding methods including the ones described herein. Dependingon the particular system, one inner codeword might contain a specificnumber (G) of outer codewords. In the outer decoder, all these Gcodewords are decoded. There are multiple possibilities that couldresult from such decoding. If decoding of all these G codewords wassuccessful, no iteration is performed and the information bits areextracted at line 3130. However, in some instances these codewords mightbe divided into two groups of G1 and G2 codewords. The codewords in thefirst group are not decoded successfully and a set of extrinsic LLRs aregenerated for each of these codewords at the output of the soft outerFEC decoder 3126 to feed back via interleaver 3112 to the inner decoder3109 as a priori information. The G2 codewords in the second group aredecoded successfully and a set of enhanced LLRs, enhanced extrinsicinformation, are generated in line 3126 using the decoded codeword bits,as explained earlier, and fed back to the inner decoder 3109 as a prioriinformation. The case G1=0 and G2=G corresponds to the first mentionedcase above. Another special case is G1=G and G2=0, where none of theouter codewords are decoded correctly. In the second iteration,generated extrinsic information on line 3126 is interleaved in 3112, ifouter interleaving/de-interleaving is employed, and the result 3127 isfed back to the inner decoder 3109 as a priori information. Using theextrinsic information from the outer FEC decoder helps in improving theperformance of the inner FEC decoder to produce more reliable softinformation for the subsequent outer FEC decoding. The iterationsbetween inner and outer FEC decoders are continued until either all theouter codewords are decoded successfully or the maximum number ofiterations is reached. As it can be seen from FIG. 18, there are twoiteration loops. One iteration loop is between the inner and outer FECdecoders 3109 and 3111, respectively, as explained above, which in someembodiments may include list decoding, i.e., passing from the inner FECdecoder to the outer FEC decoder both soft bit outputs, say LLRs, and Mmost likely hard decision sequences, and in other embodiments passingfrom the inner FEC decoder to the outer FEC decoder only soft bitoutputs. The second iteration loop is between the advanced CSIestimation 3106 and inner FEC decoder 3109. The inner FEC decoder couldalso produce inner code coded bits improved LLRs at its output 3128which are passed to the inner interleaver 3113 and interleaved coded bitLLRs 3129 are fed back to the advanced CSI estimation block 3106.Improved coded bit LLRs facilitate better channel information, includingamplitude, phase, noise and interference power, as described in variousembodiments discussed in the second subsection above pertaining to CSIestimation, and also frequency and timing estimates as it might beapplicable in some embodiments. This in turn will result in morereliable soft information at the input of the inner FEC decoder andfurther improvements in inner FEC decoding. In addition, as part of thesecond/inner loop, iterative de-mapping of noisy channel symbols intochannel bit LLRs is performed as described in other embodiments (see,e.g., inventions pertaining to check-irregular non-systematic IRAcodes). In some embodiments, the two iteration loops, inner and outer,may be performed separately and individually, i.e., each one can beexecuted without the other one. In other embodiments both inner andouter iterative loops are performed. In one embodiment, when both innerand outer loop are performed, each inner loop iteration is followed byone outer loop iteration, followed by another inner loop iteration, andso on, until maximum number of iterations is reached, or until allcodewords are correctly decoded, or until some other stopping criterionis met. In another embodiment, in case when both loops are performed,the order is as follows:

-   -   1. N₁≧1 iterations are performed between the advanced CSI        estimation 3106 (including de-mapping in 3107 and        de-interleaving in 3108) and the inner FEC decoder 3109.    -   2. The soft output of the inner FEC decoder, 3124, is        de-interleaved in 3110 and decoded using the outer FEC decoder        3111.    -   3. If all the outer codewords are decoded successfully (and pass        CRC check if it is employed), or other stopping criterion is        met, information bits are extracted at line 3130. Otherwise, the        extrinsic information from the outer FEC decoder 3126 is        interleaved in 3112 and fed back to the inner FEC decoder 3109        as a priori information.

Steps 2 and 3 above constitute one iteration of the outer iterativeloop. Up to N₂≧1 outer loop iterations are performed unless a stoppingcriterion is met.

In yet another embodiment after every N3≧1 iterations of the outer loop,one or more inner loop iterations (including advanced CSI estimation andpossibly de-mapping) are performed, followed by further N3 iterations ofthe outer loop, and so on, until maximum number of overall iterations isreached, or after another stopping criterion is met, as discussedearlier.

In yet other embodiments, the outer code is a CRC code, wherein in someembodiments one CRC code is employed for all codewords or packets of theinner code, and in other embodiments there are multiple CRC codescorresponding to different codewords or packets of the inner code.

In case that one CRC code is used for all codewords of the inner code,there are two options for decoding the outer (CRC) code. In oneembodiment, only CRC check is applied on the output of inner FEC decoder3124, or at the output of outer de-interleaver 3125 if optional outerinterleaving/de-interleaving is employed. If this output is soft LLRs,CRC check is performed on their hard decisions, and if the CRC checkpasses, the information bits are extracted from the hard decisions. Incase of list decoding of the inner code, CRC check is performed on thesequences in the output list of the list decoder. If one of thesequences in the list passes the CRC check, it will be accepted and theinformation bits are extracted from it. In both of the above cases, ifCRC does not pass, it means the decoding was not successful and softLLRs are used to extract the information bits that are not completelycorrect. Since CRC check is a hard decision process, no iteration can beperformed between the inner and outer decoders. In another embodiment,if none of the sequences or soft LLRs passes the CRC check, SISO CRCdecoding can be performed using H/M/LDPC decoding approach presented inaccordance with certain aspects of the invention or using CRC Log-MAP orother algorithms known in the art. If soft decoding resulted in acodeword, the information bits are extracted and used as the output inLine 3130. However, if soft decoding was not successful, a set ofextrinsic LLRs are generated at the output of the SISO CRC decoder.These extrinsic LLRs 3126 can be interleaved (if applicable) and fedback to the inner FEC decoder 3109 as a priori information in the nextiteration in order to help improve the inner FEC decoding performance.The iterations between inner and outer decoders are continued until asuccessful outer decoding result or until a maximum number of iterationsis reached. Similar as before, inner and outer loops may be doneindividually or they may be done together following the three stepsdiscussed above.

In case that multiple CRC codes are used for different codewords of theinner code, each inner codeword may include G CRC codewords. Followingthe same two options for outer decoding, in one embodiment only CRCcheck is applied to each CRC codeword. The G CRC codewords are dividedinto two groups, the first group with G1 codewords that have passed theCRC check and the second group with G2 codewords that have not passedthe CRC check. If G1=G and G2=0, the decoding is successful and theinformation bits are extracted at line 3130. If G1=0 and G2=G, decodingis not successful and information bits are extracted from soft LLRs andthey are not completely correct. No iteration can be performed in thiscase. However if 0<G1, G2<G, G1 CRC codewords that pass the CRC checkare converted to enhanced LLRs and fed back to the inner decoder throughinterleaver (if applicable) along with soft LLRs corresponding to G2 CRCcodewords that have not passed the CRC check for the next iteration. Itshould be noted that another iteration is only possible if at least oneof the CRC codewords passes the CRC check and can be converted toenhanced LLRs. In another embodiment, SISO CRC decoding can be appliedto G2 CRC codewords that have not passed the CRC check. In thisembodiment, G1 enhanced LLRs and G2 updated extrinsic LLRs from soft CRCdecoder are fed back to the inner FEC decoder 3109 through the outerinterleaver (if applicable) in the next iteration in order to improvethe inner decoding performance.

The fourth subsection below pertaining to advanced HD Radio decodingtechniques provides more specific examples of the use of CRC describedabove and it will be apparent to a person of skill in the art how toimplement various described combinations.

Multilevel Coding and Decoding

In another embodiment, in FIG. 18, the inner FEC code in theconcatenated scheme 3103 can be a multilevel code from the prior art.The main idea of multilevel coding is joint optimization of coding andmodulation in an effort to have the best transmission performance. Ageneral system model for multilevel encoding is shown in FIG. 19. Theset of information bits 3210 is first partitioned in 3201 into M groups,subsets. The first group 3214 is only encoded using the FEC encoder3205, while all the other groups 3211, 3212, . . . , 3213 are firstencoded using FEC encoders M−1, M−2, . . . , 1 3202, 3203, . . . , 3204and their respective coded bits 3215, 3216, . . . , 3217 are theninterleaved in blocks 3206, 3207, . . . , 3208. Output coded bits 3218and interleaved outputs 3219, 3220, . . . , 3221 go through a singlemapper 3209 where the bits are converted to modulation symbols 3222.This type of channel coding is used in Digital Radio Mondiale (DRM)standard in the prior art, for example. In DRM, different levels ofprotection can be reached for different parts of the data stream usingdifferent component codes formed with punctured convolutional codes ofdifferent rates all of which are derived from the same mother code. Butin general different codes could be used for different streams.

In addition to standard mapping, a concatenated system with multilevelcoding, such as DRM, may also use hierarchical modulation. In oneembodiment, three level coding and 64-QAM modulation are employed. Thebit stream is first divided into two parts: strongly protected part(SPP) and very strongly protected part (VSPP). The bit stream of theVSPP is sent to the encoder on level 0. The SPP is partitioned into twostreams: the bits of the higher protected part are fed to the encoder onlevel 1 and the bits of the lower protected part are fed to the encoderon level 2.

Optimal decoding of the multilevel coding scheme with an overall MaximumLikelihood (ML) or Maximum-A-Posteriori (MAP) decoder is infeasiblebecause of the very large number of states. Therefore, in certain priorart systems, suboptimum Multi Stage Decoding (MSD) is applied at thereceiver. Different levels are decoded successively starting from level0 and the estimated data are passed to all the higher level de-mappers.This process is shown in FIG. 20 using solid lines. The input in FIG. 203315 is assumed to come from the advanced CSI estimation, containing thenoisy channel symbols and CSI. This input goes to different decodinglevels via 3316, 3317, . . . , 3318. In level 0, after demapping in 3301of input symbols 3316 to coded bit LLRs 3319, the first group ofinformation is decoded using FEC decoder 0 in 3306 generatinginformation bits for the first group in line 3332. The decodedinformation from decoder 0 3324 is fed to all the higher levelde-mappers 3302, . . . , 3303. In all the other levels (1 to M−1),de-mapping in 3302, . . . , 3303 is performed on the input symbols andcorresponding CSI 3317, . . . , 3318 using the information from thedecoders of all the previous levels and coded bits are generated inlines 3320, . . . , 3321. After de-mapping, de-interleaving in 3304, . .. , 3305 is performed on the coded bits. The de-interleaved coded bits3322, . . . , 3323 are then decoded using the FEC decoders 1, . . . ,M−1 in blocks 3307, . . . , 3308. The FEC decoder outputs 3325, . . . ,3326 are then interleaved 3309, . . . , 3310 and the interleaved outputs3327, . . . go to all the higher level de-mappers. The informationpassed from one level to the other can be soft or binary. The use ofsoft information requires more complex decoders such as the Soft InputSoft Output (SISO) decoders. In case of binary information, only harddecision decoders (for example Viterbi-Algorithm for convolutionalcodes) is needed which is less complex than using a SISO decoder.Despite a higher complexity of SISO compared to decoding with harddecision outputs, by using soft information the performance of thedecoding is improved considerably. In hard decision decoding, the dataof previous levels do not include any information about the reliabilityof the decision. Therefore, the decoder in the next level has to assumethat this decision is known/correct with a probability of 1. If thisassumption is not valid, the performance of this decoder is degraded.

In order to further improve the decoding performance, iterative decodingof the individual levels may be applied in certain prior art systems.Therefore, the information obtained from all other levels can be usedfor the de-mapping and consequently decoding of a certain level. Initerative decoding, the information provided by the higher levels can beused to improve the decoding of the lower levels. The process is shownin FIG. 20 using dashed lines for the second iteration where the outputsof interleavers 3328, . . . , 3329 are fed back to all lower levelde-mappers.

In this embodiment, as mentioned earlier, the multilevel coding is usedas the inner coding 3103 in FIG. 18. In this case, the inner decoding3109 can be performed using the MSD decoding (straightforward oriterative). In case where SISO decoding is used for each decoder of theMSD, the soft LLRs at the output of the decoders (3332, 3331, . . . ,3330) are multiplexed together to form the output stream. However, incase that each decoder of the MSD uses list decoding, the output of eachdecoder includes a set of soft LLRs and a list of hard decisionsequences. The soft LLRs of all decoders are multiplexed to form theoutput LLR stream. Assuming for each of the M decoders, there is a listof M1 hard sequences, one sequence from each decoder is selected and theM sequences are multiplexed to form a final hard decision sequence.Considering all the combinations, in the end M1 ^(M) final sequences aregenerated. As said before sequences from each decoder are ordered fromthe most likely one to the least likely one. When combining sequencesfrom different decoders, different combinations have differentprobabilities of being correct. For example, the combination of firstsequences from all decoders has the highest probability of beingcorrect. The combinations are ordered based on their probabilities ofbeing correct. This way, the M1 ^(M) hard-decision sequences are alsoordered from highest probable one to the lowest probable one. So, at theoutput of the inner MSD decoder, a stream of soft LLRs along with M1^(M) binary sequences are produced.

Hierarchical Modulation

In some embodiments hierarchical modulation is employed, with examplesas mentioned previously for some of described exemplary systems. Inhierarchical modulation, two separate bit data streams are modulatedonto a single stream of modulation symbols. High Priority (HP) bitstream, or the base layer, is combined within a Low Priority (LP) bitstream, or the enhancement layer, into modulation symbols as shown inexemplary embodiments in FIG. 21 for 16-QAM and FIG. 22 for 8-PSK. As anexample, consider a hierarchical modulation with 16-QAM in FIG. 21,where both the base and enhancement layers are QPSK encoded. The baselayer is encoded with two bits and corresponds to a quadrant of the16-QAM constellation. Denser QPSK constellation in each quadrantcorresponds to two enhancement layer bits. Thus, there is largerEuclidean distance d_H 3401 for base layer bits than for the enhancementlayer bits d_L 3402. In lower SNR conditions, it may be possible todecode reliably only base layer bits. In better SNR conditions, thedetector/de-mapper can establish the phase and amplitude moreaccurately, to recover also enhancement layer bits corresponding todense QPSK clusters. In FIG. 22, for 8-PSK hierarchical modulation, thebase layer is QPSK encoded with two bits corresponding to a quadrant ofthe 8-PSK constellation. The enhancement layer is one bit correspondingto one of the two constellation points in each quadrant. Similarly, theEuclidean distance d_H 3501 for base layer bits is larger than theEuclidean distance d_L 3502 for the enhancement layer bits.

Other hierarchical symbol constellations are possible, as long as somesubsets of bits are characterized by different Euclidean distanceproperties than other subsets of bits. In other embodiments, in additionto different Euclidean distance properties of LP and HP bits, LP and HPbits could also have different levels of FEC redundancy/protection toachieve desired spectral efficiency—performance trade off. The main ideais that receivers with good reception conditions, such as good receivedSNR, can decode both streams correctly, while those with poorerreception conditions may only decode correctly the HP stream.

Hierarchical modulation could be used to facilitate robust signalreception of different users within coverage area. For example, astandard definition SDTV signal (HP stream) and a high definition HDTVsignal (LP stream) may be modulated together on the same carrier or samecomposite OFDM signal. Generally, the SDTV signal is more robust thanthe HDTV one. Depending on the quality of the received signal, the usermay be able to decode both the HDTV and SDTC streams, or the SDTV streamonly. Users near the transmission site (with good received signalstrength) or the ones with advanced receivers can correctly decode bothlayers to receive a high fidelity signal, while other users with weakersignal and/or less advanced receivers may only be able to decode baselayer, i.e., low fidelity signal.

By providing multiple layers, hierarchical modulation and coding enablesgraceful degradation in less favorable channel conditions. Hierarchicalmodulation has been included in various systems, such as DVB-T, DVB-H,MediaFLO, DVB-SH, DRM and etc., either as an option or a standardfeature. For example, in DVB-H, two options of non-hierarchical andhierarchical transmissions are included. In non-hierarchicaltransmission, the available mappings are QPSK, 16-QAM, or 64-QAM. Inhierarchical transmission only 16-QAM and 64-QAM can be used. Inhierarchical case, the inner interleaver generates its output such thateach 16-QAM or 64-QAM symbol contains 2 HP bits and its remaining bitsfrom the LP stream. In order to find the HP bits, it is sufficient thatthe de-mapper at the receiver identifies the proper quarter-plane wherethe symbol is located (by applying a QPSK de-mapping). In hierarchicaltransmission, a parameter r is defined as the minimum distance of theabove-discussed assumed QPSK points. The choices of r=1, 2, 4 arepossible. r=2, 4 results is a non-uniform QAM mapping. The larger rprovides higher protection of the HP bits.

The hierarchical modulation may be used in the general concatenatedsystem shown in FIG. 23. At the transmitter, the HP and LP bit streams3662 and 3666 go through two branches, where they are first encodedusing the outer FEC encoders 1 and 2, 3641 and 3644. The coded bits 3663and 3667 then go through outer interleavers 3642 and 3645 and theinterleaved coded bits 3664 and 3668 are encoded with inner FEC encoders3643 and 3646. The inner FEC code bits 3665 and 3669 at the outputs ofinner FEC encoders 1 and 2 go through the inner interleaver 3647 whichgenerates a single stream of interleaved coded bits 3670 such that everyS_1 HP bits are followed by S_2 LP bits that are used together toproduce a symbol carrying S_1 HP bits plus S_2 LP bits. The mapping frombits to modulation symbols in 3648 is performed such that in themodulated symbols 3671, there is larger Euclidean distance for S_1 HPbits than for S_2 LP bits.

At the receiver in FIG. 23, advanced CSI estimation 3649 is performed onthe received, noisy modulation symbols 3672 and the output 3673containing noisy channel symbols and CSI is passed to de-mapper 3650that produces coded bit LLRs 3674. Because of the larger Euclideandistance for HP bits than for LP bits, the HP bits LLRs are morereliable than LP bits LLRs. After de-mapping, inner de-interleaving 3651is applied to generate two streams of LP and HP code bit LLRs, 3675 and3679. Both streams go through inner FEC decoders 3652 and 3656 and theoutputs 3676 and 3680 are de-interleaved using de-interleavers 1 and 23653 and 3657. The outputs of de-interleavers 3677 and 3681 are thendecoded using outer FEC decoders 1 and 2 3654 and 3658 and theinformation bits or LLRs are generated in lines 3678 and 3682. In eachbranch, inner and outer FEC codes could be similar to the ones explainedin the context of FIG. 18. Iterative decoding between inner and outercodes in each branch can be done similar to the process explained forFIG. 18. There are additional loops between inner FEC decoders 1 and 2,3652 and 3656, respectively, and the CSI estimation block 3649. Eachinner FEC decoder could produce inner code coded bits improved LLRs.These coded bit LLRs 3687 and 3688 from both inner FEC decoders gothrough the inner interleaver 3661 to generate interleaved stream ofcoded bit LLRs 3689 which is fed back to the advanced CSI estimationblock 3649. Improved coded-bits LLRs from inner FEC decoders facilitatebetter estimation of channel information, including amplitude, phase,and noise and interference power, as described in various embodimentsdescribed in the second subsection above discussing CSI estimation.This, in turn, will result in more reliable soft information at theinput of both inner FEC decoders and further improvements in inner FECdecoding. In addition, as part of the loop between the advanced CSIestimation and inner FEC decoders, iterative de-mapping of noisy channelsymbols into channel bit LLRs is performed as described in otherembodiments (see e.g., inventions pertaining to check-irregularnon-systematic IRA codes).

In some embodiments, the iterations in the loops between inner and outerFEC decoders 1 and 2 (outer loops) can be performed desired number oftimes separately from the iterations in the loops between advanced CSIestimation and inner FEC decoders 1 and 2 (inner loop). For example, oneor more iterations could be performed in the inner loops betweenadvanced CSI estimation in block 3649 and inner FEC decoders 3652 and3656. One or more iterations in the inner loops may then be followed byone or more iterations in the outer loops between inner decoders 3652and 3656 and outer FEC decoders 3654 and 3658. In other embodiments, oneiteration in the inner loops is followed by one iteration in the outerloops, constituting one global iteration, and multiple global iterationsmay be performed, until maximum number of iterations is reached, oruntil all codewords for both branches are correctly decoded, or untilsome other stopping criterion is met. Various combinations of number ofiterations and interactions between inner and outer loops are possible.In one exemplary embodiment, the order is as follows:

-   -   1. N₁≧1 iterations of the inner loops are performed between        advanced CSI estimation in 3649 (including de-mapping in 3650        and de-interleaving in 3651) and inner FEC decoders 3652 and        3656.    -   2. For both branches, the soft outputs of the inner FEC encoders        3652 and 3656 are de-interleaved in 3653 and 3657 and decoded        using the outer FEC decoders 3654 and 3658.    -   3. If all the outer codewords are decoded successfully for both        branches (and pass CRC check if it is employed), or other        stopping criterion is met, information bits are extracted at        lines 3678 and 3682. Otherwise, the extrinsic information from        the outer FEC decoders 3683 and 3685 are interleaved in blocks        3659 and 3660 and the results 3684 and 3686 are fed back to the        inner FEC decoders 1 and 2 as a priori information.

Steps 2 and 3 above constitute one iteration of the outer iterativeloops. Up to N₂≧1 outer loop iterations are performed unless a stoppingcriterion is met.

In yet another embodiment after every N3≧1 iterations of the outerloops, one or more inner loop iterations (including CSI estimation andpossibly de-mapping) are performed, followed by further N3 iterations ofthe outer loop, and so on, until maximum number of overall iterations isreached, or after another stopping criterion is met, as discussedearlier.

In one embodiment similar inner FEC encoding in 3643 and 3646 is appliedto both HP and LP parts of the stream. In another embodiment, differentlevels of FEC protection is applied to the HP and LP bits, in additionto different distance properties of the hierarchical constellation, toprovide desired level of performance optimization.

A more detailed implementation of some aspects of the inventionregarding concatenated coding and CSI estimation will be explained inthe context of exemplary embodiments for HD Radio decoding in the nextsubsection. Implementation of various other embodiments described inthis subsection should be apparent to those skilled in the art, fromdescriptions in this subsection as well as description of variousdescribed embodiments in other subsections.

IV. Advanced HD Radio Decoding

This subsection discusses certain embodiments for decoding HD Radiosignals, including hybrid HD Radio signals. Some of the embodimentsconcentrate on decoding of the system control data sequences carried byreference subcarriers in FM HD Radio systems. Specifically, certainaspects of the invention discussed herein pertain to providingintegrated, iterative receivers/decoders that improve the performance ofFM HD Radio systems, through techniques for soft-diversity combining ofsoft values of repeated control data bits carried on a plurality ofreference subcarriers, techniques for collectively utilizing softcombining and multiple symbol detection, and/or techniques forcorrecting bit errors using parity check bits and utilizing thecorrected parity bits in the techniques of soft-diversity combining aswell as in the techniques of collectively utilizing soft combining andmultiple symbol detection. These improvements not only help to decodecontrol data sequences more reliably, but also facilitate better CSIestimation that employs reference subcarriers, which contributes toimproved decoding of all HD Radio logical channels.

In addition, other improvements including, improved and iterative CSIestimation in conjunction with soft-input soft-output list Log-MAPdecoding of the employed convolutional codes, as well as improvedsoft-input soft-output RS decoding, for decoding of HD Radio logicalchannels are described in various embodiments. These improvements resultin extending the range/coverage of digital radio by correspondingimplementations in HD Radio receivers, without modifying the existing HDRadio system infrastructure or the air-interface. As discussed furtherbelow, most of the principles discussed in this subsection are alsoapplicable to non-HD Radio systems and standards.

FIG. 24a illustrates the functionality of a transmitter that multiplexesa plurality of information sequences into a single stream in the timedomain as applicable to HD Radio systems including AM and FM, as well ashybrid and all-digital. While certain specific numerology and details ofdifferent versions/variations of HD Radio systems are omitted forsimplicity, some relevant ones are included and/or pointed out.

Information Source 1 4002 assembles a series of Main Program Service(MPS) Protocol Data Units (PDUs) on line 4033 each of which includes aplurality of audio packets and a program service data (PSD) PDU. Theinput to the Audio Encoder 4003 is a stream of audio frames generated byan Audio Interface running at a typical sample rate for audio, e.g.,44.1 kilo-samples per second (kSa/s), for left and right audio channels.The Audio Encoder breaks each audio channel into segments, known asaudio frames, processes each segment, and delivers encoded audio packetson line 4028. Each encoded audio packet is then processed by CRC Encoder4004 for integrity check at the receiver. PSD is generated in block 4005for transmission along with the program audio. PSD may be placed in aspecific format to form a PDU and delimited by one-byte flag. The PSDPDU is then processed by CRC Encoder 4006 for integrity check at thereceiver. MPS PDUs are generated in block 4007 with a fixed headerportion (i.e., Control Word), a variable header portion (including avariable number of audio packet location fields and an optional headerexpansion field), a PSD PDU, and encoded audio packets. The Control Wordis protected by a RS (96,88) code in block 4008. Since the RS codewordis of fixed size, i.e., 96 bytes, it may span portions of the headerexpansion field, PSD PDU field, and possibly a fraction of the encodedaudio packet bits. The rest of the MPS PDU bits beyond 96 bytes remainunchanged in block 4008. The output from the RS Encoder 4008 forms alogical channel, e.g., Logical Channel P1 in HD Radio systems. The bitstream of the logical channel is processed by Convolutional Encoding4009 with a tail-biting convolutional code. Convolutional encoding mayinclude mother code generation, e.g., code rate 1/3, and puncturing,which together determine the code rate for the logical channel, e.g.,code rate 2/5 for some modes of operation in HD Radio systems. The codedbit stream on line 4034 is further bit-interleaved. But thisbit-interleaving in block 4010 may be performed in association withanother interleaver 4015 for a coded bit stream from another logicalchannel, e.g., Primary IBOC Data Service (PIDS) used in HD Radiosystems, when the two logical channel bits are multiplexed (i.e., block4035) to form a single bit stream on line 4036.

The coded bit stream on line 4039 is generated from Information Source 24011 which is for Station Information Service (SIS) in HD Radio systems.The output from SIS Data Generator 4012 is processed by CRC Encoding4013 for integrity check at the receiver. The output on line 4038 fromthe CRC Encoder forms a logical channel, e.g., Logical Channel PIDS inHD Radio systems. The bit stream of the logical channel is processed byConvolutional Encoding 4014 with a tail-biting convolutional code.

Information Source 3 4016 assembles a series of Advanced ApplicationService (AAS) PDUs for fixed and/or opportunistic data as in HD Radiosystems. AAS Data Generator 4017 receives AAS Data from the ServiceInterfaces and then encodes and encapsulates the data to generate AASpackets. Each AAS packet is processed by CRC Encoder 4018. FEC may beapplied to the encoded packet stream on line 4042 to control packet lossand errors using the following methods: RS (255,223) block coding inblock 4019 for error correction, byte interleaving in block 4020 toprotect against error bursts, and block synchronization mechanism whichis omitted for simplicity in the diagram. The output on line 4044 formsa logical channel, e.g., Logical Channel P3 (or P4 if an additional dataprocessing is performed, i.e., Information Source 4, as in LogicalChannel P3) in HD Radio systems. The bit stream of the logical channelP3 (and/or P4) is processed by Convolutional Encoding 4021 with aconvolutional code. Convolutional encoding may be performed with thesame mother code as the one for Logical Channel P1 but with a differentpuncturing pattern to result in a different code rate, e.g., code rate½. The coded bit stream on line 4045 is then bit-interleaved in block4022. As indicated above, in another embodiment, an additionalinformation source, i.e., Information Source 4 for Logical Channel P4(not shown), could be present in HD Radio systems. In this case,additional blocks for convolutional encoding (block 4021) andinterleaver (block 4022) are added to process the Logical Channel P4 bitstream.

System Control Data Sequence Assembler 4023 processes System ControlChannel (SCCH) information into a set of system control data sequences.In the FM HD Radio system, there are 61 system control data sequences of32 bits long in a processing block denoted by matrix Rd (or also knownin the art as matrix r (lowercase)), and each output sequence on line4047 is carried by one of the 61 reference subcarriers that aredistributed throughout the OFDM spectrum. The Differential Encoder 4024then differentially encodes each 32-bit sequence in time in the FM HDRadio system. With all 61 sequences differentially encoded and furthertransposed, the resulting output on line 4048 is a matrix R (uppercase)of fixed dimension 32×61. The row dimension of R (i.e., 32) correspondsto the number of OFDM symbols per pre-defined time duration and thecolumn dimension (i.e., 61) corresponds to the maximum number of activereference subcarriers per OFDM symbol. In the AM HD Radio system, SystemControl Data Assembler 4023 processes SCCH information along withsynchronization, parity and reserved bits into a stream of systemcontrol data sequences. The resulting output on line 4047 is a columnvector R destined for two reference subcarriers with BPSK modulation inthe AM HD Radio system. Thus, block 4024 is not applicable to the AM HDRadio system.

Symbol and OFDM Subcarrier Mapping 4025 assigns the interleaver matriceson line 4036 for Logical Channels P1 and PIDS, referred to as matrix PMin the FM HD Radio system, and on line 4046 for Logical Channel P3,referred to as matrix PX1 (and matrix PX2 for Logical Channel P4, ifpresent) in the FM HD Radio system, and the system control matrix R online 4048 to the OFDM subcarriers. The inputs to OFDM Subcarrier Mappingare a row of each active interleaver/system control matrix which isprocessed every OFDM symbol duration (i.e., Ts) to produce an outputvector, herein referred to as X which is a frequency-domainrepresentation of the signal. The output vector X from Symbol and OFDMSubcarrier Mapping for each OFDM symbol is a complex vector of length1093. The vector is indexed from k=0, 1, 2, . . . , 1092. The k-thelement of X corresponds to subcarrier (k−546). Symbol and OFDMSubcarrier Mapping 4025 first maps bits into modulation symbols. Forinstance, bits read from interleaver partitions, PM, PX1 (and PX2) inthe FM HD Radio system, and individual bits read from R are mapped tocomplex modulation-constellation values, and the appropriate amplitudescaling factor is applied to these complex constellation values. In AMHD Radio systems, interleaver matrices multiplexing and mapping tosubcarriers are somewhat different and with different notation, butwould be well understood by persons of ordinary skill in the art, andsuch details are omitted for brevity. Such modulation symbol mapping fordata subcarriers may be performed for different modulations schemes suchas QPSK, 16-QAM and 64-QAM in different modes of HD Radio systems. Forexample, for QPSK modulation, the following mapping may be employed:

I Bit Q Bit Constellation Value 0 0 (−1 − j1) 0 1 (−1 + j1) 1 0 (1 − j1)1 1 (1 + j1)

For reference subcarriers, symbol mapping of the bits in matrix R may beby the following rules:

Bit Value Constellation Value 0 (−1 − j1) 1 (1 + j1)

Then, block 4025 maps the scaled complex constellation values to theappropriate elements of the output vector X. Elements of X correspondingto unused subcarriers are set to the complex value 0+j0.

The OFDM Signal Generation 4026 receives complex, frequency-domain, OFDMsymbols from the Symbol and OFDM Subcarrier Mapping, and outputstime-domain signal representing the digital portion of the FM (or AM orall digital) HD Radio signal. The OFDM signal generation is achieved byusing the inverse discrete Fourier transform (IDFT). Also, the guardinterval αT, where α is the cyclic prefix width, e.g., 7/128, and T=1/Δfis the reciprocal of the OFDM subcarrier spacing, is placed in thebeginning of the OFDM symbol duration T_(S). Not shown in the figure forsimplicity, the output on line 4050 from the OFDM Signal Generationmodulates radio-frequency (RF) carrier in a method well known in the artand is transmitted through the radio channel 4027. Being transmittedthrough the radio channel, the transmitted signal may suffer frommultipath fading commonly encountered in the real-world radiotransmission as well as other impairments, such as various forms ofinterference. The output on line 4051 of the radio channel is receivedand processed by the receiver 4060 in FIG. 24 b.

In the receiver 4060 shown in FIG. 24b , Receiver Front-end 4061processes the received radio signal (on line 4051 in FIG. 24a ) with themethods commonly adopted in the prior art for carrier demodulation,including but not limited to carrier recovery and timingsynchronization, and appropriate sampling, i.e., analog-to-digitalconversion, ultimately resulting in a discrete baseband signal, andoutputs the baseband signal on line 4074. Then, OFDM De-modulation andSubcarrier De-mapping 4062 performs the Fourier transform and subcarrierde-mapping to produce two streams of baseband signals, one on line 4077for information sources 1, 2, and 3 and the other on line 4075 for thesystem control data, respectively. The system control data bits arefirst decoded in block 4063 from the system control data signal on line4075 to produce regenerated system control data bits as well as rawsystem control data signal on line 4076. Using the regenerated systemcontrol bits and raw system control data signal, the Advanced CSIEstimation block 4064 performs channel state information (CSI)estimation, which produces the channel response and noise powerestimates that is further explained herein, and performs phasecorrection, i.e., coherent demodulation, of the signal on line 4077. Theresulting signal on line 4078 is a stream of phase-corrected complexsymbols. Symbol-to-bit De-mapping 4065 calculates LLRs from the symbolson line 4078 and assigns them to corresponding coded-bit positions.These LLRs on coded-bit positions are also referred to as channel LLRs.Then, the channel LLRs on line 4079 are de-interleaved and demultiplexedin block 4066 to produce the logical channel signal P1 (i.e., channelLLRs corresponding to the coded-bit positions of logical channel P1) online 4080, the logical channel signal PIDS on line 4081, and the logicalchannel signal P3 on 4082 as well as the logical channel signal P4 asapplicable (not shown in FIG. 24b ). These logical channel signals arethen processed by SISO decoders, in blocks 4067, 4068, and 4069,respectively, for decoding of the logical channel bits that werechannel-encoded with a convolutional code at the transmitter. SISOdecoding could be performed using Log-MAP decoders, or varioussub-optimum implementations such as Max-Log-MAP or soft-output Viterbialgorithm (SOVA), soft-output sequential decoding or other “tree” basedalgorithms, or other algorithms known in the art, or in some steps couldbe implemented using list Log-MAP algorithm as explained in someembodiments and as it will be discussed subsequently for specificimplementations in HD Radio systems. Then, to improve the CSI estimationand decoding accuracy, the output LLRs on line 4083, 4084, and 4085,which are coded bit LLRs, from the SISO/Log-MAP decoders may beprocessed in the Interleaver and MUX block 4070. In block 4070, allthree (or four if Logical Channel P4 is present) logical channel signalsare interleaved and multiplexed in the same way as processed in thetransmitter, i.e., block 4040 in FIG. 24a . Then, its output signal online 4089 is supplied to Advanced CSI Estimation 4064 for anotheriteration of CSI estimation as described in previous embodiments. Signalwith improved CSI estimates is passed subsequently to blocks 4065-4069for next decoding iteration. As discussed in the previous embodiments,there are multiple alternatives to implement joint iterative decodingand CSI estimation and the number of iterations depends on the stoppingcriterion. In one of alternative approaches, after the final iterationof joint CSI estimation and SISO decoding in the loop including AdvancedCSI Estimation 4064 and SISO Decoders 1, 2 and 3, 4067-4069, the SISOdecoders, employing a list Log-MAP algorithm, may produce both theoutput LLRs and M most likely hard-decoded sequences, as explainedearlier, on line 4086, line 4087, and line 4088, corresponding toinformation-bit positions of the logical channels for further decodingof the information, i.e., MPS PDUs in block 4071, SIS PDUs in block4072, and AAS PDUs in block 4073, respectively. More details of certainrelevant aspects of the invention are further described below.

The methods described herein can be applied to both FM and AM, hybridand all-digital HD Radio systems taking into considerationsystem-specific parameters such as constraint length of theconvolutional code, e.g., 7 for FM and 9 for AM, code rate, or number ofsubcarriers, modulation and other elements of numerology. Detailednumerology for individual modes of HD Radio systems is omitted as itwould be well understood by persons of ordinary skill in the art, andwhen specific numerology is relevant it will be taken into account.

Control Channel Decoding

In reference to block 4063 in FIG. 24b , the methods constructed in thisembodiment are based on diversity combining of soft values of thedifferentially encoded control data sequence bits (or modulation-symbolsas applicable throughout certain embodiments) to enhance the detectionof the system control data sequence of HD Radio systems. To furtherelaborate the methods, consider FIG. 25 which is a simplifiedrepresentation of the transmitter 4001 in FIG. 24a for the FM HD Radiosystem with additional logical channel P4 from Information Source 4 aswell as interleaver matrices, PM, PX1, PX2, and R denoted accordinglyalong with their illustrative dimensions. Of particular importance isthe Control Data Sequence Assembler 4104 which produces a set of logicalbits denoted by matrix Rd (also interchangeably referred to as thebinary reference matrix) on line 4116. The size of Rd is (P×M), forexample, P=61 and M=32 as shown in the figure. The M-bit sequence ineach row of Rd is differentially encoded and transposed, and P sequencescollectively produce a matrix R (also interchangeably referred to as Rmatrix) on line 4112 of size (M×P). Note that the row dimension of Rd(or the column dimension of R, i.e., P=61) corresponds to the maximumnumber of active reference subcarriers per OFDM symbol and the columndimension of Rd (or the row dimension of R, i.e., M=32) corresponds tothe number of OFDM symbols per pre-defined time duration. All or some,say U, of the M-bit sequences in a (P×M) matrix Rd may contain the samebit pattern (or patterns) (such as SYNC bits and/or Control bits shownin FIG. 28) at designated bit positions in each sequence. Furthermore, aplurality of (P×M) Rd matrices may be concatenated to produce longersequences in rows of Rd, resulting in a (P×qM) Rd matrix if concatenatedq times, and thus qM-bit sequences in columns of R. In this case, thesame bit pattern (or patterns) placed in all (e.g., U=P) or some (e.g.,U<P)) of the sequences in a (P×M) Rd matrix appear q·U times in theconcatenated (P×qM) Rd matrix. For example, with q (P×M) Rd matricesconcatenated, a resulting ((q·M)×P) R matrix with q=32, M=32, and P=61is shown on line 4112 in FIG. 25. The output, R matrix, of theDifferential Encoding 4105 is processed in the Symbol and OFDM Mappingblock 4106 as described in earlier embodiments.

FIG. 26 is an alternative block diagram illustrating the functionalityof the receiver in FIG. 24b in accordance with certain embodiments. Thebaseband signal on line 4128 (equivalent to the signal on line 4074 inFIG. 24b ) is supplied to OFDM Demodulation and Subcarrier De-mapping4121. Its output on line 4129 is supplied to Advanced Decoding of R4122. The R matrix decoding block extracts a set of complex symbolsdenoted by R′ corresponding to the R matrix in FIG. 25 as well as matrixR′_(d). The R′ matrix on line 4132 is supplied to Advanced CSIEstimation 4124 for CSI estimation and phase correction of the signal online 4134 carried on data subcarriers (i.e., information bits fromInformation Sources). Advanced CSI Estimation 4124 may also performfrequency offset estimation for the entire OFDM subcarriers as well astiming estimation of the system control data sequence. The decodedoutput R′_(d) matrix is supplied to Advanced Decoding of System ControlData Sequence 4123 for further processing. The processing in block 4123may include regeneration of R matrix using R′_(d) matrix and its output4133 may be fed-back to the Advanced CSI Estimation block 4124 forimprovement of CSI/time/frequency estimation. Symbol-to-bit De-mapping4125 calculates channel LLRs from the symbols on line 4135 and assignsthem to corresponding coded-bit positions. The channel LLRs are furthersupplied to the De-MUX block 4126 to produce the interleaver matrices ofchannel LLRs on lines 4137, 4138, and 4139. The interleaver matrices arethen de-interleaved by corresponding logical channel interleavers, i.e.,Type I for P1, Type II for PIDS, and Type IV for P3 and P4, followed bySISO decoding in block 4127.

FIG. 27 is a schematic diagram detailing the operation of the OFDMsignal generation block 4107 in FIG. 25. The modulated data symbols inblock 4141 represent the QPSK symbols on the data streams 4109, 4110,4111 in FIG. 25. The data symbols on line 4142 are, after symbol andOFDM subcarrier mapping, supplied to the Inverse Fast Fourier Transform(IFFT) block of length N 4146. Similarly, the R_(d) matrix whichcontains control data sequence bits to facilitate channel estimation,synchronization, as well as other control functions for the overall HDRadio system operation, is formed in block 4143 and modulated by theDPSK modulator in block 4144. The DPSK modulated control symbols on line4145, equivalent to R matrix on line 4112 in FIG. 25 are supplied to theIFFT block 4146. Both the data and control data symbols (the latter willbe used in the receiver to facilitate block synchronization and channelestimation, among other functions) are processed by the IFFT of length Nin block 4146 to transform the data and control symbol sequences intothe time domain. The resulting time domain signal is also supplied toblock 4147 in which a suffix, whose length is chosen to be longer thanthe expected delay spread, e.g., 7/127 of the OFDM symbol durationT_(S), to avoid the inter symbol interference (ISI), is formed. Theoutputs of block 4146 and block 4147 are processed by the parallel toserial (P/S) block 4148 to form a set of OFDM symbols and converted toan analog OFDM signal before transmission on the RF carrier (not shown).

OFDM signal is transmitted over a multipath channel (block 4149 in FIG.27) and then noise is added (block 4150), representing thermal noise inthe receiver and possibly other sources of interference. After initialfrequency and timing synchronization and frequency down conversion (RFdemodulation), the received signal is sampled and processed by serial toparallel (S/P) block 4151 and its output is supplied to block 4152 andblock 4153 where removal of the suffix part and fast Fourier transform(FFT) take place, respectively, to transform the data and controlsequences back into the frequency domain samples. The output of the FFTblock on line 4154 is supplied to block 4155 to form the receivedDPSK-modulated signal matrix R, denoted at the receiver as R_(rec) thatwill be used to decode the system control data sequence matrix R′_(d) inthe Advanced Decoding of R block 4156. A prior art method or preferablycertain embodiments may be applied in block 4156 to decode theDPSK-modulated matrix R_(rec) that carries necessary information todecode the data coherently in block 4159. The output of the FFT block online 4158 is also supplied to block 4159 to recover the data symbolsutilizing the signal on line 4157 from the block 4156.

FIG. 28 shows a schematic representation of the M-bit system controldata sequence (SODS). In HD Radio systems, M=32 but a different value ofM could be used as applicable to a radio system design. This systemcontrol data sequence also corresponds to one of the P rows of R_(d)matrix on line 4116 in FIG. 25. The system control data sequenceincludes synchronization, control, parity, and reserved bits. As shownin FIG. 28, eleven (11) out of thirty two (32) bits represent the knownsynchronization sequence (SYNC) and the SYNC bits are placed in fields4176, 4173, 4169, and 4165, and used for block synchronization andchannel estimation purposes. The system control data sequence alsoincludes control bits in fields 4162, 4164, 4167, 4171, and 4172, andreserved bits in fields 4163, 4168, and 4175 that carry informationabout transmit block counts and other information pertinent to the framestructure that carries the data sequences 4109, 4110, and 4111 in FIG.25. Moreover, the system control data sequence also includes four paritybits in fields 4161, 4166, 4170, and 4174 to protect bits in the parityfields 4177, 4178, 4179, and 4180, and possible inventive use of theparity bits in HD Radio systems will be discussed subsequently.

TABLE 10 SCDS Bits Bit Positions U Synchronization 0/1/2/3 31-25, 22,17, 10-9 30 Reserved 2/1/0 24, 16, 7 30 Parity 3/1/0 23, 11, 0 30Control 4/3/2/1 19, 15-12, 8, 6-1 30 Control 5 21-20 6/8/8/8 Parity2 186/8/8/8

When one or more particular fields (or bit patterns) are repeatedly usedin U system control data sequences in a processing block, i.e., (P×M)matrix R_(d), the transmitted bits corresponding to those fields (or bitpatterns) in the system control data sequence are repeated (i.e.,carried) on U reference subcarriers. Table 10 shows an example of thisrepetition of particular fields or bit patterns in the system controldata sequence for a case of thirty (30) reference subcarriers, i.e.,P=30, as used in the Primary Service Mode of operation in the hybrid FMHD Radio system. As shown in the table, in this example, most bits inthe system control data sequence are repeated over all subcarriersexcept for Control 5 bits in the field 4172, where the referencesubcarrier identification (RSID) bits are transmitted in the HD Radiosystem. In these Control bits, their protection bit, Parity 2 in thefield 4170, may be different over their respective referencesubcarriers. However, clearly, in the FM HD Radio system, frequencydiversity is provided to those particular fields and/or bit patterns bytransmission of the identical modulated symbols on multiple referencesubcarriers and/or subbands. Likewise, with the concatenation of aplurality (e.g., q) of (P×M) R_(d) matrices that are transmitted on q·MOFDM symbols, time diversity is provided to those particular fieldsand/or bit patterns in a sequence over q consecutive (P×M) blocks intime.

FIG. 29 shows a mapping of system control data sequence bits (ormodulation symbols after symbol mapping) onto a set of referencesubcarriers 4192 and a plurality of OFDM symbols 4191. The control datasequence bits (or modulation symbols) are carried on the referencesubcarriers (shaded ones) distributed over the OFDM spectrum (on thefrequency axis). In reference to certain embodiments mentioned earlier,there are P (=up to 61) reference subcarriers and those P referencesubcarriers are distributed throughout the OFDM spectrum. In thisexemplary embodiment, the mapping of system control data sequence bits(or modulation symbols) onto reference subcarriers resembles the methodin extended hybrid spectrum of the HD Radio FM air interface thatutilizes the primary sidebands within its defined spectrum band.

The binary reference matrix R_(d) on line 4116 in FIG. 25 can beexpressed as

$\begin{matrix}{R_{d} = \begin{bmatrix}d_{1,1} & d_{1,2} & \ldots & d_{1,{M - 1}} & d_{1,M} \\d_{2,1} & d_{2,2} & \ldots & d_{2,{M - 1}} & d_{2,M} \\\vdots & \vdots & \ddots & \vdots & \vdots \\d_{{P - 1},1} & d_{{P - 1},2} & \ldots & d_{{P - 1},{M - 1}} & d_{{P - 1},M} \\d_{P,1} & d_{P,2} & \ldots & d_{P,{M - 1}} & d_{P,M}\end{bmatrix}} & (31)\end{matrix}$where indexes P and M correspond to the number of reference subcarriersand the total number of OFDM symbols, respectively. As mentionedearlier, some elements of the R_(d) matrix are repeated in thepre-defined rows to enhance the performance at the receiver. Then, eachrow of matrix R_(d) is DPSK-modulated and transmitted on one of thereference subcarriers 4191 in FIG. 29.

Regarding the DPSK modulation of matrix R_(d), the differentiallyencoded sequence c_(p,m) is generated from the elements of R_(d),{d_(p,m)}, by taking a modulo-2 addition of d_(p,m) and the immediatelypreceding encoded bit, c_(p,m−1) and can be written asc _(p,m) =d _(p,m) ⊕c _(p,m−1)  (32)where ⊕ denotes the modulo-2 addition. The constellation for theresulting modulated signal element, R_(p,m), of the R matrix on line4112 in FIG. 25 (or equivalently, on line 4145 in FIG. 27) is given by

$\begin{matrix}{R_{p,m} = \{ {\begin{matrix}{- 1} & {{{if}\mspace{14mu} c_{p,m}} = 0} \\1 & {{{if}\mspace{14mu} c_{p,m}} = 1}\end{matrix}.} } & (33)\end{matrix}$

In reference to FIG. 26 and FIG. 27, the received OFDM signal on line4128 in FIG. 26 is demodulated by the FFT block 4153 in FIG. 27 and theDPSK-modulated reference symbols 4154 in FIG. 27 are extracted from thereference subcarriers to form R_(rec) and written as

$\begin{matrix}{R_{rec} = \begin{bmatrix}r_{1,1} & r_{1,2} & \ldots & r_{1,{M - 1}} & r_{1,M} \\r_{2,1} & r_{2,2} & \ldots & r_{2,{M - 1}} & r_{2,M} \\\vdots & \vdots & \ddots & \vdots & \vdots \\r_{{P - 1},1} & r_{{P - 1},2} & \ldots & r_{{P - 1},{M - 1}} & r_{{P - 1},M} \\r_{P,1} & r_{P,2} & \ldots & r_{P,{M - 1}} & r_{P,M}\end{bmatrix}} & (34)\end{matrix}$where r_(i,j), i=1, . . . , P and j=1, . . . , M, are complex numbers.The purpose of decoding matrix R blocks, 4122 and 4123, in FIG. 26 (orequivalently, the block 4156 in FIG. 27) is to obtain the R_(d) matrixat the receiver that will be further utilized for CSI estimation andsynchronization in block 4124 in FIG. 26.

The following is intended to describe processes for decoding of thesystem control data sequence bits using soft diversity combiningaccording to certain principles of the invention. To help providecontext for such processes, an exemplary method that is known in the artis first described. Such a method is based on majority logic combiningof hard decisions on individual reference subcarriers, as described inU.S. Pat. No. 7,724,850. This technique is herein referred to as theMajority Voting Combining (abbreviated as “MVC”). In reference to theprocessing flow shown in FIG. 30, it can be further summarized asfollows.

Step 1: From the received demodulated OFDM signal on line 4207, form theR_(rec) matrix (block 4201).

Step 2: Construct a shifted version of R_(rec) (block 4202) denoted asR_(rec-1) such that the output on line 4209 can be written as

$\begin{matrix}{R_{{rec} - 1} = {\lbrack {{I( {P,1} )},{R_{rec}( {:{,{{1\text{:}M} - 1}}} )}} \rbrack = {\begin{bmatrix}{- 1} & r_{1,1} & \ldots & r_{1,{M - 2}} & r_{1,{M - 1}} \\{- 1} & r_{2,1} & \ldots & r_{2,{M - 2}} & r_{2,{M - 1}} \\\vdots & \vdots & \ddots & \vdots & \vdots \\{- 1} & r_{{P - 1},1} & \ldots & r_{{P - 1},{M - 2}} & r_{{P - 1},{M - 1}} \\{- 1} & r_{P,1} & \ldots & r_{P,{M - 2}} & r_{P,{M - 1}}\end{bmatrix}.}}} & (35)\end{matrix}$where I(P,1) is a P×1 zero vector with its elements being a logical bitvalue of 0 mapped onto amplitude of −1.

Step 3: Obtain the element-wise product matrix R_(s) (block 4203) byR _(s) =Re{R _(rec-1) *·R _(rec)}  (36)where ‘●’ denotes the operation of element-by-element multiplication andRe{□} is the operator to select the real part of the complex number.Note that in (35) a commonly accepted notation x(m: n) is used toextract entries m through n of vector x, and a notation X(m₁:n₁, m₂:n₂)is to extract the sub matrix from rows m₁ through n₁ and from columns m₂through n₂. The initial value of DPSK modulated signal is set to “−1”for each reference subcarrier. The product matrix R_(s) can be furtherwritten as

$\begin{matrix}{{R_{s} = {{Re}\{ \lbrack \begin{matrix}{{- 1^{*}}r_{1,1}} & {r_{1,2}^{*}r_{1,1}} & \ldots & {r_{1,{M - 2}}^{*}r_{1,{M - 1}}} & {r_{1,{M - 1}}^{*}r_{1,M}} \\{{- 1^{*}}r_{2,1}} & {r_{2,2}^{*}r_{2,1}} & \ldots & {r_{2,{m - 2}}^{*}r_{2,{M - 1}}} & {r_{2,{M - 1}}^{*}r_{2,M}} \\\vdots & \vdots & \ddots & \vdots & \vdots \\{{- 1^{*}}r_{{P - 1},1}} & {r_{{P - 1},2}^{*}r_{{P - 1},1}} & \ldots & {r_{{P - 1},{M - 1}}^{*}r_{{P - 1},{M - 1}}} & {r_{{P - 1},{M - 1}}^{*}r_{{P - 1},M}} \\{{- 1^{*}}r_{P,1}} & {r_{P,2}^{*}r_{P,1}} & \ldots & {r_{P,{M - 1}}^{*}r_{P,{M - 1}}} & {r_{P,{M - 1}}^{*}r_{P,M}}\end{matrix} \rbrack \}}},} & (37)\end{matrix}$and with d_(p,m) ^(s) defined as the (p,m)^(th) element of R_(s) fornotational simplicity, it is further written as

$\begin{matrix}{R_{s} = \begin{bmatrix}d_{1,1}^{s} & d_{1,2}^{s} & \ldots & d_{1,{M - 1}}^{s} & d_{1,M}^{s} \\d_{2,1}^{s} & d_{2,2}^{s} & \ldots & d_{2,{M - 1}}^{s} & d_{2,M}^{s} \\\vdots & \vdots & \ddots & \vdots & \vdots \\d_{{P - 1},1}^{s} & d_{{P - 1},2}^{s} & \ldots & d_{{P - 1},{M - 1}}^{s} & d_{{P - 1},M}^{s} \\d_{P,1}^{s} & d_{P,2}^{s} & \ldots & d_{P,{M - 1}}^{s} & d_{P,M}^{s}\end{bmatrix}} & (38)\end{matrix}$

Step 4: Obtain hard decision on each element of the R_(s) matrix (block4204) as follows:

$\begin{matrix}{d_{p,m}^{h} = {d_{p,m}^{s}\overset{m_{1}}{\underset{m_{2}}{\gtrless}}0}} & (39)\end{matrix}$where m₁ and m₂ are the decision values of d_(p,m) ^(h) that correspondto bit “0” and bit “1”, respectively. The four steps above leading to(39) are indeed for non-coherent differential demodulation. Thenon-coherent differential demodulation is employed in the prior art asthe entries in R_(rec) correspond to the entries of the DPSK-modulatedsignal matrix R in the transmitter.

Step 5: For the m-th control sequence bit repeated U_(m) times andplaced on U_(m) reference subcarriers, majority voting 4205 (which isreferred to as majority voting combining) is performed after findinghard decisions on all elements of R_(s). Generally, U_(m) is an oddnumber for better facilitation of the majority voting. The finaldecision for the (p,m)^(th) element, d_(p,m) ^(hd), is given by

$\begin{matrix}{d_{p,m}^{hd} = {( {d_{p_{1},m}^{h} + d_{p_{2},m}^{h} + \ldots + d_{p_{Um},m}^{h}} )\overset{m_{1}}{\underset{m_{2}}{\gtrless}}\frac{U_{m} + 1}{2}}} & (40)\end{matrix}$where the term in the right-hand side is the threshold for the majorityvoting which would be normally an odd number if U_(m) is an odd number,and the decision values m₁ and m₂, are for bit 0 and bit 1,respectively, and pε{p_(m)}, p_(m)=p₁, p₂, . . . p_(u) . . . p_(U) _(m)with p_(u) denoting the subcarrier index for one of U_(m) subcarriersamong the P reference subcarriers in the system. Although (40) could becalculated for all U_(m) subcarriers, as the U_(m) results would be thesame, it can be performed only once for a subcarrier in the set {p_(m)}.In an alternative embodiment of the prior art method, if U_(m) is aneven number, a round-up (i.e., ceil( )) or down (i.e., floor( ))operation could be performed on the threshold value in the right-handside, i.e., round-up/down

$( \frac{U_{m} + 1}{2} ),$or simply, a tie could be resolved by randomly selecting a value, i.e.,by flipping a coin.

Step 6: Then, the same bit decision on line 4212 is applied in block4206 to all U_(m) subcarriers that carry one of the repeated controlsequence bits at its m-th bit position.d _((pεp) _(m) _(),m) ^(h) =d _(p,m) ^(hd)  (41)

For one of the embodiments, FIG. 31 illustrates the processing flow fordecoding of the system control data sequence bits with soft diversity(abbreviated as ‘sd’) combining with non-coherent DPSK. Soft diversitycombining will facilitate better non-coherent DPSK decoding of controldata sequence. As shown in FIG. 31, Steps 1 through 3, 4221, 4222, 4223,may correspond to the first three steps shown in FIG. 30, which areapplied to find the R_(s) matrix defined in (38) such that the matrix isoutput on line 4230. In the next step, Step 4, ‘soft’ values in U_(m)subcarriers carrying the repeated system control data sequence bit aresummed in block 4224 as follows

$\begin{matrix}{d_{p,m}^{sc} = ( {d_{p_{1},m}^{s} + d_{p_{2},m}^{s} + \ldots + d_{p_{Um},m}^{s}} )} & (42)\end{matrix}$where the superscript ‘sc’ represents soft combining. In the next step,Step 5, after the soft combining, hard decision on the (p,m)^(th)element of R_(s) is obtained in block 4225 by

$\begin{matrix}{d_{p,m}^{h} = {d_{p,m}^{sc}\overset{m_{1}}{\underset{m_{2}}{\gtrless}}0}} & (43)\end{matrix}$where m₁ and m₂ are decision values of d_(p,m) ^(h) that correspond tobit “0” and bit “1”, respectively, as in the prior art method. Finally,the same bit decision on line 4232 is applied in block 4226 to all U_(m)subcarriers that carry one of the repeated bits at its m-th bitposition, which is mathematically represented as follows:d _((pεp) _(m) _(),m) ^(h) =d _(p,m) ^(h)  (44)where pε{p_(m)} with p_(m)=p₁, p₂, . . . p_(u) . . . p_(U) _(m) as inthe prior art method (40) mentioned earlier.

For another embodiment shown in FIG. 32, the decoding of the systemcontrol data sequence bits is further extended to exploit multiplesymbol detection in addition to soft diversity combining. The method(MVC) in FIG. 30 and soft diversity combining (sd) embodiment in FIG. 31use the conventional non-coherent DPSK demodulation where the actualphase is determined by the difference between the received phases of twoconsecutive symbols. Multiple-symbol differential detection (MSDD)method by Divsalar and Simon, which employs more than two consecutivesymbols for DPSK demodulation, provides better error rate performancethan the conventional DPSK demodulation. In MSDD, the received carrierphase is assumed to be constant over the time duration corresponding tothe plurality of consecutive symbol intervals utilized for detection. Inanother embodiment shown in FIG. 32, the decoding of the system controldata sequence bits employs soft diversity combining and multiple symboldetection (abbreviated as “sdm”). As an example, as the frequencyresponse of the wireless channel may change over time, three consecutivesymbols are used for the MSDD. More specifically, in reference to FIG.32, “sdm” is performed in the following steps:

Step 1: From the received demodulated OFDM signal on line 4252, form theR_(rec) matrix (block 4241) as shown in (34).

Step 2: Construct a shifted version of R_(rec) denoted by R_(rec-1)(block 4242) and output on line 4254, written as

$\begin{matrix}{R_{{rec} - 1} = {\lbrack {{I( {P,1} )},{R_{rec}( {:{,{{1\text{:}M} - 1}}} )}} \rbrack = {\quad\begin{bmatrix}{- 1} & r_{1,1} & \ldots & r_{1,{M - 2}} & r_{1,{M - 1}} \\{- 1} & r_{2,1} & \ldots & r_{2,{M - 2}} & r_{2,{M - 1}} \\\vdots & \vdots & \ddots & \vdots & \vdots \\{- 1} & r_{{P - 1},1} & \ldots & r_{{P - 1},{M - 2}} & r_{{P - 1},{M - 1}} \\{- 1} & r_{P,1} & \ldots & r_{P,{M - 2}} & r_{P,{M - 1}}\end{bmatrix}}}} & (45)\end{matrix}$where I(P,1) is a P×1 zero matrix with each logical bit value of 0mapped onto amplitude of −1.

Step 3: Calculate another shifted version of R_(rec) denoted byR_(rec-2) (block 4243) and output on line 4255, written as

$\begin{matrix}{R_{{rec} - 2} = {\lbrack {{Z( {P,1} )},{I( {P,1} )},{R_{rec}( {:{,{{2\text{:}M} - 1}}} )}} \rbrack = {\quad\begin{bmatrix}0 & {- 1} & r_{1,2} & \ldots & r_{1,{M - 1}} \\0 & {- 1} & r_{2,2} & \ldots & r_{2,{M - 1}} \\\vdots & \vdots & \ddots & \vdots & \vdots \\0 & {- 1} & r_{{P - 1},2} & \ldots & r_{{p - 1},{M - 1}} \\0 & {- 1} & r_{P,2} & \ldots & r_{P,{m - 1}}\end{bmatrix}}}} & (46)\end{matrix}$where Z(P,1) denotes a P×1 zero vector.

Step 4: The MSDD for DPSK in this embodiment is based onmaximum-likelihood sequence estimation (MLSE) for three symbolintervals. This method results in four phase differential sequences.Calculate R_(C) _(x) , x=1, 2, 3, 4 (block 4259) using the followingequations for the four cases.

$\begin{matrix}{R_{C_{x}} = {{{Re}\{ {( {R_{rec} \cdot R_{{rec} - 1}^{*}} ){\mathbb{e}}^{- {j\Phi}_{k}}} \}} + {{Re}\{ {( {R_{{rec} - 1} \cdot R_{{rec} - 2}^{*}} ){\mathbb{e}}^{- {j\Phi}_{k - 1}}} \}} + {{Re}\{ {( {R_{rec} \cdot R_{{rec} - 2}^{*}} ){\mathbb{e}}^{- {j{({\Phi_{k} + \Phi_{k - 1}})}}}} \}}}} & (47)\end{matrix}$where C₁={Φ_(k)=0, Φ_(k-1)=0}, C₂={Φ_(k)=0, Φ_(k-1)=π}, C₃={Φ_(k)=π,Φ_(k-1)=π}, C₄={Φ_(k)=π, Φ_(k-1)=0}. This results in R_(C) ₁ 4245,R_(C2) 4246, R_(C3) 4247, and R_(C4) 4248 that are output on line 4256and can be expressed, respectively, asR _(C) ₁ =Re{R _(rec) ·R _(rec-1) *}+Re{R _(rec-1) ·R _(rec-2) *}+Re{R_(rec) ·R _(rec-2)*}  (48)R _(C) ₂ =Re{R _(rec) ·R _(rec-1) *}−Re{R _(rec-1) ·R _(rec-2) *}−Re{R_(rec) ·R _(rec-2)*}  (49)R _(C) ₃ =−Re{R _(rec) ·R _(rec-1) *}−Re{R _(rec-1) ·R _(rec-2) *}+Re{R_(rec) ·R _(rec-2)*}  (50)R _(C) ₄ =−Re{R _(rec) ·R _(rec-1) *}+Re{R _(rec-1) ·R _(rec-2) *}−Re{R_(rec) ·R _(rec-2)*}  (51)

Denote the (p,m)^(th) elements of R_(C) ₁ , R_(C) ₂ , R_(C) ₃ , R_(C) ₄, respectively, as r_(p,m) ^(c1), r_(p,m) ^(c2), r_(p,m) ^(c3), r_(p,m)^(c4).

Step 5: For the m-th symbol, repeated U_(m) times and carried on thesubcarriers pε{p_(m)} where p_(m)=p₁, p₂, . . . p_(U) _(m) , calculatethe metrics r_(p,m) ^(c1), r_(p,m) ^(c2), r_(p,m) ^(c3), r_(p,m) ^(c4)of soft diversity combining over the subcarriers in block 4249 for thefour cases with

$\begin{matrix}{r_{p,m}^{c\; 1} = ( {r_{p_{1},m}^{c\; 1} + r_{p_{2},m}^{c\; 1} + \ldots + r_{p_{U_{m},}m}^{c\; 1}} )} & (52) \\{r_{p,m}^{c\; 2} = ( {r_{p_{1},m}^{c\; 2} + r_{p_{2},m}^{c\; 2} + \ldots + r_{p_{U_{m},}m}^{c\; 2}} )} & (53) \\{r_{p,m}^{c\; 3} = ( {r_{p_{1},m}^{c\; 3} + r_{p_{2},m}^{c\; 3} + \ldots + r_{p_{U_{m},}m}^{c\; 3}} )} & (54) \\{r_{p,m}^{c\; 4} = ( {r_{p_{1},m}^{c\; 4} + r_{p_{2},m}^{c\; 4} + \ldots + r_{p_{U_{m},}m}^{c\; 4}} )} & (55)\end{matrix}$

Equations (52-55) represent the probabilities of the defined cases.Among the (p,m)^(th) elements of R_(C) ₁ , R_(C) ₂ , R_(C) ₃ , and R_(C)₄ , select the index of the maximum value (block 4249) given as[r _(p,m) ^(c) i _(x)]=max{r _(p,m) ^(c1) ,r _(p,m) ^(c2) ,r _(p,m)^(c3) ,r _(p,m) ^(c4)}  (56)where r_(p,m) ^(c) and i_(x) denote the maximum value among r_(p,m)^(c1), r_(p,m) ^(c2), r_(p,m) ^(c3), r_(p,m) ^(c4) and its index invector {r_(p,m) ^(c1), r_(p,m) ^(c2), r_(p,m) ^(c3), r_(p,m) ^(c4)},respectively.

Step 6: Then, with the index on line 4257, the final decision ford_(p,m) is made as follows:d _(p,m)=0 if i _(x)=1 or 2  (57)d _(p,m)=1 if i _(x)=3 or 4  (58)

Step 7: Apply the same decision value of d_(p,m) on line 4258 to therest of the elements representing the repeated symbols (block 4251):d _((p) _(ε) _(p) _(m) _(),m) =d _(p,m)  (59)

Certain aspects of the invention also incorporate a method of correctingthe (even or odd) parity bit in each parity field by bit flipping when apredefined criterion is satisfied. One embodiment incorporating the bitflipping adopts a conventional bit flipping in the prior art. In theprior art, the parity bit for each parity field is calculated at thereceiver and compared with the received parity bit. If they are thesame, it is assumed that there is no bit error in the parity field ofthe decoded system control data sequence (as an even or odd parity codecan correctly detect only single-bit errors), otherwise the parity fieldis assumed to be corrupt. When there is an error in the parity field,the error will propagate into the rest of the sequence when differentialencoding is performed on the decoded system control data sequence toregenerate a DPSK signal. To prevent such error propagation caused bythe single-bit error in the parity field, the received parity bit can beflipped when it does not match the calculated parity bit. Thisconventional parity flipping (abbreviated as “cpf”) is applied to theaforementioned embodiments, i.e., those abbreviated as “sd”, and “sdm”,respectively, as well as the prior art method abbreviated as “MVC”, toenhance the system performance. The resulting embodiments are denoted asMVC with conventional parity flipping (MVC-wcpf), sd with conventionalparity flipping (sd-wcpf), and sdm with conventional parity flipping(sdm-wcpf), respectively.

Another embodiment incorporating the parity bit flipping utilizes thereliabilities of parity-field bits to select a bit to be flipped in eachparity field. In this aspect of the invention, the least reliable bit ineach parity field is flipped. More specifically, according to certainembodiments, for an example of an even parity code, the R_(d) matrix isformed at the transmitter (block 4104 in FIG. 25) with the systemcontrol data sequence shown in FIG. 28 that includes parity bits 4161,4166, 4170, and 4174. The parity bit in a parity field can be denoted byd_(p,m+D) and written asd _(p,m+D) =d _(p,m) ⊕d _(p,m+1) ⊕ . . . ⊕d _(p,m+D−1)  (60)where d_(p,m+j), j=1, . . . , D−1 denote the parity-field source bits ina parity field covered by the even parity code. Then, as shown in FIG.33, at the receiver 4260, the R_(rec) matrix is formed (block 4261) asin other embodiments. Subsequently, hard bit-decisions are made on theparity-field bits and denoted by P_(f) ^(h)=[d_(p,m) ^(h), d_(p,m+1)^(h), . . . , d_(p,m+D−1) ^(h), d_(p,m+D) ^(h]) (block 4262) and itsreliabilities of soft value are denoted by P_(f) ^(s)=[d_(p,m) ^(s),d_(p,m+1) ^(s), . . . , d_(p,m+D−1) ^(s), d_(p,m+D) ^(s]) (block 4263).With the hard bit-decisions on line 4271, it is checked whether or noteven parity is satisfied in the parity field (block 4264). For the evenparity check, the following criterion is used:if [d _(p,m) ⊕d _(p,m+1) ⊕ . . . ⊕d _(p,m+D−1) ⊕d _(p,m+D)]=0  (61)

If even parity is satisfied (or equivalently, the parity check passes)4272, it is assumed that no errors were made in the transmission of theparity-field code word and vector P_(f) ^(h) on line 4271 of harddecisions of the transmitted sequence is used as the true transmittedvector (block 4265). If even parity is not satisfied 4273, whichindicates that there is one error, or an odd number of errors ingeneral, in the code word, then, the bit with the smallest reliability,most likely to be in error, among those in the code word is flipped.More details of this error correction procedure by parity bit-flippingare described in the following steps 1-2.

Step 1: Find the absolute values of all elements in P_(f) ^(s) (block4266) and output the result P_(f) ^(a) on line 4275, written asP _(f) ^(a) =[abs(d _(p,m) ^(s)),abs(d _(p,m+1) ^(s)), . . . ,abs(d_(p,m+D−1) ^(s)),abs(d _(p,m+D) ^(s))]  (62)

Step 2: Find the index of the smallest value in P_(f) ^(a) (block 4267)that is an identification of the least reliable bit in the vector:k=arg min{P_(f) ^(a)}  (63)

Equation (63) returns the index k of the minimum value in the vector. Byusing the index k, the k-th element of the P_(f) is flipped in block4268, which can be mathematically written asP _(f)(k)=P _(f)(k)⊕1  (64)

Flipping the least reliable bit (abbreviated as “flr”) is then appliedto the aforementioned embodiments, abbreviated as “sd” and “sdm” toenhance the system performance. The resulting embodiments are denoted assd with parity check (sd-wflr) and sdm with parity check (sdm-wflr),respectively.

R-matrix decoding performance of certain embodiments was evaluated viacomputer simulation to illustrate and confirm certain benefits of theinvention. The parameters of the fading channel models used to evaluateand compare the performance of the conventional method and variousembodiments are summarized for urban slow (USLOW) and urban fast (UFAST)in Table 11.

TABLE 11 Urban Fast Rayleigh Multipath Urban Slow Rayleigh MultipathProfile (UFAST) Profile (USLOW) Attenua- Delay Doppler Attenuation DelayDoppler tion Ray (ms) (Hz) (dB) Ray (ms) (Hz) (dB) 1 0.0 0.1744 2.0 10.0 5.2 2.0 2 0.2 0.1744 0.0 2 0.2 5.2 0.0 3 0.5 0.1744 3.0 3 0.5 5.23.0 4 0.9 0.1744 4.0 4 0.9 5.2 4.0 5 1.2 0.1744 2.0 5 1.2 5.2 2.0 6 1.40.1744 0.0 6 1.4 5.2 0.0 7 2.0 0.1744 3.0 7 2.0 5.2 3.0 8 2.4 0.1744 5.08 2.4 5.2 5.0 9 3.0 0.1744 10.0 9 3.0 5.2 10.0

FIG. 34 and FIG. 35 compare the BER and the FER performance of themajority voting combining with conventional parity flipping (MVC-wcpf)receiver with certain embodiments abbreviated as sd-wcpf, sdm-wcpf,sd-wflr, and sdm-wflr over the USLOW fading channel model, respectively.In these two figures, it is clearly shown that these embodimentsoutperform the prior art method referred to as the MVC-wcpf. It is alsonoted that the detection gain (i.e., a difference in Eb/No value in dBto get the same BER) in these embodiments over the MVC-wcpf increases asthe Eb/No (or SNR) increases.

Moreover, for certain embodiments based on soft diversity combining(i.e., “sd”) and soft diversity combining and multiple symbol detection(i.e., “sdm”), flipping the least reliable bit (i.e., “flr”) is moreeffective in improving the decoding performance than the conventionalparity flipping (i.e., “cpf”). It is also noted that the sd-wflr andsdm-wflr have comparable performance while the sd-wflr has lowercomplexity than sdm-wflr. The observed gains from the computersimulation of certain embodiments for the R matrix decoding aresummarized in Table 12 in comparison to the conventional method.

TABLE 12 MVC-wcpf sd-wcpf sdm-wcpf sd-wflr sdm-wflr BER = 1e−5 4.2 dB4.9 dB 5.3 dB 5.4 dB FER = 1e−3 5.5 dB 6.3 dB 7.0. dB 7.0 dB

The conventional method and certain embodiments are also compared interms of BER and FER in the UFAST channel in FIG. 36 and FIG. 37,respectively. In comparing FER curves in USLOW and UFAST, it is notedthat the decoding performance of the prior art method and certainembodiments slightly degrade in the UFAST channel. However, it isclearly shown that certain embodiments sustain the detection gain overthe MVC-wcpf even in the UFAST channel at the desired FER of 10⁻³. Theobserved gains from the computer simulation of certain embodiments forthe R matrix decoding are summarized in Table 13 in comparison to theconventional method.

TABLE 13 MVC-wcpf sd-wflr sdm-wflr sd-wflr sdm-wflr BER = 1e−5 6.2 dB5.6 dB 8.7 dB 8.25 dB FER = 1e−3 5.9 dB 6.3 dB 7.5 dB 7.4 dBApplication to R-Matrix Decoding in AM HD Radio Systems

Both the FM HD Radio systems and AM HD Radio systems have a similarsystem control data sequence structure of length M (=32) bits wheresynchronization and parity fields are the same in the M-bit sequence.However, in reference to FIG. 25, R=R_(d) in the AM HD Radio systems asdifferential encoding is not employed. Also, the R matrix is a columnvector of 256 bits as eight (8) M (=32)-bit column vectors areconcatenated. Considering the symbol mapping onto BPSK signalconstellation by the following rule:

Bit 0→0−j0.5 Bit 1→0+j0.5

and OFDM subcarrier mapping onto two subcarriers as

Subcarrier No. −1: −R*Subcarrier No. 1: R

one can note that the system control data sequences on the two carriersare the same, i.e., elements of −R* and R. As such, the repetitionfactor U of the system control data sequence over the frequency domainis 2 for all bits in the sequence (while it could be much larger,depending on the field in the sequence, for the example of PrimaryService Mode in the FM HD Radio system as illustrated earlier in Table).

In this case, for the decoding of R_(d) in block 4123 of FIG. 26, upperand lower reference subcarrier are first equal gain combined by takinginto account negative complex conjugation of the lower referencesubcarrier. Next, in one embodiment, multiple symbol detection is alsoperformed, with an option to perform additional combining over two ormore subsequent blocks in the time domain. In other embodiments, the“conventional parity flipping” and preferably the “flipping the leastreliable bit” are applicable to the AM HD Radio systems. It should beclear to those skilled in the art that although detailed description isomitted, the decoding of R (=R_(d)) matrix doesn't involve non-coherentdecoding in block 4122 in FIG. 26 as differential encoding is notemployed at the transmitter and coherent demodulation is facilitated bythe presence of AM carrier and other auxiliary and control signals.

Channel Estimation in HD Radio Systems

Specifics of CSI estimation in HD Radio systems depend on the particularversion, such as all-digital or hybrid, AM or FM. Without loss ofgenerality, consider a hybrid FM mode illustrated in FIG. 38. In thisexample, the digital signal is transmitted in selected lower and upperprimary sidebands as 4801 and 4802, respectively, while analog FM signal4803 is shown in the middle. Each sideband contains multiple frequencypartitions. One frequency partition 4804 is defined as a group of 19OFDM subcarriers containing 18 data subcarriers 4805 and one referencesubcarrier 4806 for control/synchronization purposes.

Each reference subcarrier 4806 in HD Radio system carries differentiallyencoded system control data sequences. Some fields of the control datasequence are known, such as SYNC bits, while others may carry controlinformation that may repeat over subcarriers and/or consecutive blocksin time, as described in the previous embodiments. N referencesubcarriers are distributed across OFDM spectrum, wherein N depends on aspecific version and a mode.

The system control data sequence received on the reference subcarriersis decoded at the receiver by using the known structure of thetransmitted signal according to inventive methods described in previousR-matrix embodiments. It was shown in FIG. 34-FIG. 37 that suchinventive methods perform much better than prior art methods, typicallyby as much as 10 dB in the range of lower BERs. Such decoded andreconstructed reference subcarrier symbols are then employed as the“pilot” signals to facilitate CSI estimation according to the inventivemethods of advanced CSI estimation embodiments. The specificconstellation of reference subcarriers in HD Radio systems is analogousto the dedicated subcarriers case in FIG. 10a . The principal differenceis that in HD Radio systems case, some “pilot” symbols on the referencesubcarriers are known while others are reconstructed based on advanceddecoding methods described earlier and could exhibit occasionalerroneous reconstructed “pilot” symbols, which in turn may have anegative impact on CSI estimation.

Specifically for the hybrid FM case, CSI estimation is performed usingthe dedicated pilot scenario as described previously in the variousembodiments of advanced CSI estimation. First, initial advanced CSIestimation is performed in accordance with the algorithm depicted byFIG. 15, with the corresponding descriptions, and LUTs Table 6-Table 9.Median and smoothing filters are applied to reduce the impact ofadditive noise and interference, host FM and possibly first adjacent FMinterference. Iterative CSI estimation is also employed in accordancewith the structure in FIG. 16 and corresponding descriptions. It isverified by simulations for a slow urban channel model that the decodedFERs of audio and data HD Radio channels, by employing advanced CSImethods described herein (e.g., in the second subsection), show similarand superior performances for cases of perfectly known pilots andreconstructed pilots decoded using the inventive methods or R-matrixembodiments. In contrast, using conventional single stage CSI estimationmethods, such as described in U.S. Pat. Nos. 6,549,544, 7,724,850, witha fixed filter lengths suitable for a range of channel scenarios, theFER performance with reconstructed pilots may be as much as 0.5-1.0 dBworse than assuming perfectly known pilots, depending on a particularscenario.

In one embodiment, CSI estimation in AM HD Radio systems is performedusing the AM carrier, reference subcarriers and training bits insertedinto interleaved data sequences. CSI estimation on the AM carrier may bedone by methods well known in the art. Known decoded bits on referencesubcarriers are used also for CSI estimation in accordance with previousembodiments. Similarly, CSI estimation on training bits transmitted ondata subcarriers are decoded in accordance with the embodiments ofadvanced CSI estimation. Aggregate use of all these components enablesbetter CSI estimation.

Decoding Information Source 1 (Logical channel P1)

As shown in FIG. 39a , the MPS PDU 4401 from Information Source 1consists of a number of groups of information bits, e.g., MPS PDU header4402, audio packets 4404, 4405, 4406, and program service data 4403. TheMPS PDU header field 4402 contains necessary control information forproper processing of the PDU and covered by an appropriate RS code,e.g., (96,88). Depending on the length of the RS code block 4407, somepart of the PSD field 4403 may be included in the RS code block. The RSparity bytes are placed in the beginning of the MPS PDU.

The PSD field 4403 begins with a flag 4411 to indicate its beginning andthe PSD control 4412 and PSD Payload 4413 are protected by CyclicRedundancy Check (CRC) parity bits field 4414. Within an MPS PDU, theremay be a plurality of audio packets, e.g., 1-n. The audio packet lengthmay be different depending on the audio codec used. Each audio packet isprotected by its CRC parity bits field, e.g., 4416, 4418, or 4420.

As shown in FIG. 39b , the SIS PDU 4431 consists of various fields4433-4441, and these fields are protected by CRC parity bits field 4442.The length of SIS PDUs may be relatively short compared to the length ofthe MPS PDU and is typically fixed, e.g., 80.

FIG. 40 shows one embodiment for decoding of MPS PDU header. The signalon line 4495 in FIG. 40 is a stream of channel LLRs, which is also shownon line 4080 in FIG. 24b . After de-puncturing of the LLR stream inblock 4481, i.e., inserting zero values in positions where the codedbits were punctured in the transmitter to bring the coded stream to theoriginal code rate before puncturing, e.g., 1/3, the LLR sequencecorresponding to the MPS PDU header, which corresponds to a RS codewordof length 96 bytes, is extracted in block 4482 from the stream on line4496. This RS codeword output on line 4497 is decoded in block 4483using a list Log-MAP decoding algorithm. The list Log-MAP decoder blockproduces a set of output LLRs, both information and coded bit LLRs,i.e., systematic and parity bits of the RS (96,88) codeword, and apredefined number of most likely hard-decision sequences (containing 0'sand 1's) 4498 corresponding to the RS codeword. Such a predefinednumber, referred to as M_value, could be any integer larger than orequal to 2 (The special case M_value=1 corresponds to no list decoding).By increasing M_value, the performance of list decoding improves, butthere is a diminishing return beyond a moderate M_value, say 32. Inspecific simulation examples of MPS PDU header decoding, M_value=32 wasemployed.

For the case of tail-biting convolutional code employed in HD Radiosystems, tail-biting list Log-MAP decoding is employed in 4483. Sincefor tail-biting convolutional codes the initial encoder state is equalto the ending encoder state, Head and Tail bit sequences are used tofacilitate tail-biting decoding. Specifically, for a given coded bitsegment that is being decoded, say, C={c₁, C₂, . . . , c_(t), c_(t+1), .. . , c_(N-h), c_(N-h+1), . . . , c_(N-1), c_(N)}, Head and Tail bitsequences, respectively, are added before bit c₁ and after bit c_(N),respectively. Head and Tail bits are given by H={c_(N-h+1), . . . ,c_(N)} and T={c₁, . . . , c_(t)}, such that the following sequence isbeing decoded

={H, C, T}. For initialization of forward recursions in Log-MAPalgorithm, from the first bit of Head sequence toward the last bit ofTail sequence, the trellis states at the beginning of the Head sequenceare initiated with equal probabilities. For example, for a convolutionalcode with memory m, there are 2^(m) states and each state is assignedprobability ½^(m). Similarly, for initialization of backward Log-MAPrecursions, starting from the last bit of Tail sequence toward the firstbit of Head sequence, all states are assigned same probability ½^(m) atthe end of Tail sequence. The lengths of Head and Tail sequences, h andt, respectively, are selected to be several constraint lengths of theconvolutional code. For example, for a convolutional code of constraintlength 7, selection h=t=50 provides good results, such that nonoticeable gain is observed by increasing their lengths beyond 50. Thisis motivated by the fact that a convolutional decoder converges to thecorrect sequence within few constraint lengths, which was exploited inthe art previously to use a finite decoder memory in decoding ofconvolutional codes. Using the described Head and Tail approach, thedecoding complexity for tail-biting decoding, relative to nontail-biting decoding, is increased by the factor 1+(h+t)/N, whichbecomes negligible for N>>h+t. As a point of reference, MATLABtail-biting Viterbi algorithm requires processing proportional to 2·N,while still underperforming the aforementioned list Log-MAP tail-bitingdecoder by about a quarter dB at lower BER values, based on simulationresults for fading channels.

List decoding is implemented according to the methods of Lanneman andSundberg. In one embodiment, optimal generation of the list of mostlikely sequences is employed. In another embodiment, a suboptimum methodis used. The list of length M_value of hard decision sequences isordered such that the most likely sequence is the first on the list,next most likely is the second on the list, etc. In simulation resultsshown later, the suboptimum list generation method was used. By usingthe optimum list generation method, a small improvement in performancemay be achieved in fading channels, although in AWGN channels theimprovement is negligible with sufficiently large M_value.

Block 4484 checks, starting with the first entry in the list ofhard-decision sequences on line 4498, each sequence to determine if thesequence is a RS codeword. That is, the RS (96,88) code is employed forerror detection. If a valid RS codeword is declared, the codeword online 4499 is further passed to block 4485 to check if the decodedsequence is a valid MPS PDU header by checking the consistencyconditions for various fields in the MPS PDU header. The consistencycheck provides an additional level of error detection capability beyondthat provided by the RS (96,88) code. The consistency check is performedby exploiting the deterministic relationship of some fields in theheader with the corresponding fields in the previous radio frame, e.g.,PDU Sequence Number in the Fixed Header part 4409 shown in FIG. 39aincreases by one from one frame to the next frame, and similarly forother fields of the Fixed Header 4409. Also, the consistency checkexploits the relationship between different fields within one MPS PDUheader. For example, locator fields defined in the HD Radio standard,NOP_Lc, pointing to byte positions that delineate consecutive audiopackets in the P1 logical channel of a radio frame must satisfy thefollowing relationship, NOP_Lc(i)<NOP_Lc(i+1). In another embodiment,the consistency check could be omitted, since RS (96,88) has fairly gooderror detection capability, depending on the desired level ofprobability of undetected errors. If the sequence represents aconsistent header it is passed on line 4500 to block 4492 to extractrelevant information bits on line 4514 and it is also passed to block4493 to produce enhanced LLRs on line 4513 which are passed to block4494 to combine them with LLRs from other parts of the packet and toproduce output LLRs on line 4515, if required for iterative decoding oriterative CSI estimation. Enhanced LLRs are simply hard decisions, +1 or−1, multiplied by a relatively large number. Enhanced LLRs are used foriterative decoding when the corresponding segment is declared correct,and enable to give more weight to such bit LLRs than to LLRs from othersegments which may not have been declared correct and where normal LLRswould be used, which will slightly improve iterative decoding or CSIestimation.

If none of the valid sequences on line 4498 is a valid RS codeword,hard-decision RS decoding based on the Berlekamp Massey (BM) algorithmis performed on at least L_(BM) of the M_value sequences in block 4486where L_(BM) is an integer less than or equal to M_value. Similarly, ifnone of valid RS codewords on line 4499 passes the consistency check inblock 4485, the set of M_value sequences is passed to block 4486 for BMdecoding as described above. If BM decoding was successful on any ofthese L_(BM) sequences, the resulting highest ranked RS codeword on line4504 is fed to block 4488 to see if it is a valid MPS PDU header. If so,information bits are extracted from it to line 4514 and it is alsoconverted to enhanced LLRs on line 4513 to be used as the output LLRs online 4515.

However, if none of the results is a valid MPS PDU header (line 4507) orBM decoding was not successful on any of the L_(BM) sequences (on line4505), the LLRs produced by Log-MAP corresponding to the RS codewordbits are fed to a soft-input soft-output (SISO) RS decoder, sometimesalso referred to as ‘soft’ RS decoder, in block 4489. Operation andperformance of soft RS decoder is explained in detail in previousembodiments. If soft RS decoding resulted in a valid RS codeword (online 4509), it is further tested in block 4491 to see if it is a validMPS PDU header. If so, information bits are extracted from it on line4514 and it is also converted to enhanced LLRs on line 4513 and used asthe output LLRs on line 4515. If the resulting RS codeword was not avalid MPS PDU header (on line 4511), the input LLRs to the soft RSdecoder are passed to blocks 4492 and 4494 to extract information bitsand output LLRs from them, respectively. When the soft RS decoder inblock 4489 did not result in a valid RS codeword (on line 4512), itoutputs a set of updated LLRs based on soft RS decoding employing BPdecoding, as described in the corresponding embodiments. These LLRs areused in blocks 4492 and 4494 to extract information bits and output LLRsfrom them, respectively. The output LLRs may be used for furtheriterative processing shown in FIG. 46.

Since the MPS PDU header contains important information about the systemand its information bits are carried on logical channel P1, it isimportant to decode this header as accurately as possible. In oneembodiment, the MPS PDU header decoding is further improved byexploiting deterministic or probabilistic relationships of differentfields either over consecutive frame intervals, or between consecutivefields. By exploiting the structure of particular fields, it is possibleto improve the corresponding bits prior to RS decoding of the header. Bycorrecting some bit errors in this manner, the error correction burdenon the RS code is reduced and it may correct more erroneous code wordson balance.

For example, several fields in the fixed part of the MPS PDU header inHD Radio systems, such as Stream ID, Blend Control, Latency, etc. arerarely changed and could be assumed to be constant from frame to frame.Thus, if the MPS PDU header is decoder correctly in frame (i−1), i.e.,no errors are detected by the RS code, the corresponding fields could beassumed to take the same value in frame i, thus removing possible biterrors in some of the positions of these fields. Understandably, whenone of these fields is changed, which occurs very rarely, the proposedapproach will introduce an error even if the received bits of thecorresponding field are correct. However, assuming that the frequency ofchanges is much slower than the bit error rate, the proposed approach isstill beneficial for performance improvements. Alternatively, tominimize the adverse impact of assuming the field value from theprevious frame, the field value from the previous frame could beassigned a probability that is proportional to the probability that thefield will take same value in the next frame, while other possible fieldvalues could be assigned correspondingly smaller probabilities.

Some fields have deterministic relationship from frame to frame. Forexample, PDU Sequence Number in the fixed header part is increased by 1from frame (i−1) to frame i, modulo the sequence number range. Thus,first time the MPS PDU header is decoded correctly, this field could beassumed to be known and to be deterministically changing from frame toframe.

Other fields may have different types of relationship. For example audiopackets locator fields, referred to as Loc in the HD Radio standard,point to the last byte (CRC byte location) of the audio packet. Sincethere is a relatively large number of Loc fields, e.g., 24 to 40,possibly containing 16 bits each, it is very important to improve thereliability of these fields/bits prior to RS decoding. To facilitatethat, one could first notice that the next Loc field takes larger valuethan the previous Loc field, that is Loc(i)< Loc(i+1), i=0, 1, . . . ,NOP-2, where NOP is the total number of audio packets in the frame, ingeneral variable from frame to frame. This introduces a memory in thesequence of Loc fields, enabling to preprocess it with a trellis basedalgorithm. Although each 16-bit field in general could take any of 2¹⁶possible values, thus suggesting a trellis with 2¹⁶ states, not allstates will be possible. For example, consider the stage j of thetrellis corresponding to Loc(j) and consider the m-th state for Loc(j).By exploiting the property that Loc(j)< Loc(j+1), it is clear that stagem of Loc(j) could only have transitions to states (m−1), (m+2), . . . ,2¹⁶ of Loc(j+1). Thus, this results in a variable trellis withprogressively reduced number of trellis branches/transitions.

Furthermore, the audio packets could be characterized by definingminimum and maximum audio packet lengths, Nmin and Nmax, respectively,determined by measurements of various audio traffic types and codecrates audio samples. Such information could be passed to the MPS PDUdecoder as side information. This information may help further todrastically reduce the number of states in the trellis. Now the range oftransitions from state m of Loc(j) to the states of Loc(j−1) is reducedfrom (m+1)˜2¹⁶ to (m+Nmin)˜(m+Nmax), which will reduce trelliscomplexity significantly and also improve the decoding gain due to moreconstrained trellis, although occasionally the trellis description maynot be complete when the actual packet length is either smaller thanNmin or larger than Nmax. Correspondingly, reduced trellis complexityand larger preprocessing gain could be achieved by tightening the rangeNmin−Nmax, but with increased probability of incomplete trellisdescription which may cause an error floor at low bit error rates. Thus,a desired balance between the gain at low SNR and error floor at highSNR could be achieved by the system designer as desired.

Additional improvement could be achieved by exploiting some additionalproperties of the audio stream. For example, certain number of bits atthe beginning of each audio frame may be constant in each audio frame,which could be provided from the audio codec as side information to theMPS PDU decoder. Such a priori knowledge of certain bits at thebeginning of each audio packet can further be exploited to introducebias of transitions through the trellis. For example, let us assume thatL bits, b_1, . . . , b_L, at the beginning of each audio packet areknown. Then, for each state in the trellis of Loc(j), one could examinethe corresponding L bits in the audio bit sequence following the end ofpacket j indicated by the state of Loc(j), and calculate the probabilitythat they take values b_1, . . . , b_L. These probabilities, in turn,could be appropriately related to the corresponding states of Loc(j),thus making some states more likely than others and in that way furtherimproving decoding of MPS PDU header. One could think of this approachas using a “diversity” approach. That is, decoding of the sequence ofbits in MPS PDU header is improved by using a different sequence ofbits, audio bits, by exploiting side information available for certainaudio bits at the beginning of each audio frame, whereas the beginningof audio frame is related to the Loc field of the previous audio packet.

Thus, by taking soft bits from the SISO convolutional decoder output,say provided in a form of LLRs, the described trellis structure andproperties could be utilized by a SISO variable trellis algorithm, suchas Log-MAP, e.g., to produce improved LLRs that will be furtherprocessed by the RS decoder and enable it to achieve lower probabilityof incorrect decoding. By employing described techniques noticeableperformance gain in decoding of MPS PDU header could be achieved.

Processing of PSD

FIG. 41 shows one embodiment for decoding of PSD PDUs. From a stream ofchannel LLRs for an MPS PDU on line 4528, which is also shown on line4496 in FIG. 40, a PSD PDU is extracted in block 4521. Since thelocation of PSD PDU may not be fixed within the MPS PDU, its location issearched based on known information. One embodiment may includesearching the FLAG bit pattern such as PSD Flag 4411 in FIG. 39a bycross-correlating the hard decoded sequence, using a sliding window.With the known FLAG pattern, select the position that maximizes thecorrelation peak. Another embodiment may use LLRs instead of harddecoded sequence to maximize the correlation peak, corresponding toproper alignment of the FLAG pattern.

Once the PSD PDU is found, list Log-MAP decoding is performed in block4522 using as input the channel LLRs corresponding to the PSD PDU bits.The list Log-MAP decoder outputs bit decisions and the output LLRs ofthe information bits in the PSD PDU as well as a set (i.e., list) ofpath sequences, that is, hard-decision sequences, of binary value of 0or 1 on line 4530. Then, CRC check is performed on the maximum aposteriori probability (MAP) bit decisions of the PSD PDU in block 4523,as known in the art. If the PSD PDU passes the CRC check 4531, its bitdecisions are output as the final bit decisions in block 4529 for theinformation bits of the PSD PDU and each LLR value is enhanced in 4524to a large value of magnitude preserving its polarity, e.g., 100 or −100for logical bit 1 or 0 (or vice versa, depending on the binary bitmapping in the system). If the MAP PSD PDU sequence fails CRC check4532, the M_value path sequences of the PSD PDU from the list decoderare then CRC-checked one at a time in block 4523 in the order they areplaced in the list, until a path sequence in the list passes the CRCcheck. Some path sequences in the list may be the same and thus anyduplicated path sequences may be removed before running this CRC checkin order to reduce the processing time required to perform thisoperation. When the first path sequence is found that passes the CRCcheck 4523, the path sequence is output as the final bit decisions 4529of the information bits of the PSD PDU and their LLR values are enhanced4524 as described earlier. If no path sequence is found 4532 that passesthe CRC check in block 4523, in one embodiment CRC Log-MAP decoding isperformed 4526 on the decoder-output LLRs from block 4522. Then, theLLRs for the information bits of the PSD PDU are determined from theoutput LLRs of the CRC Log-MAP decoder (on line 4537) and passed to line4538. Also, the final bit decisions for the information bits of the PSDPDU are made in block 4529 on the polarity of the output LLRs of the CRCLog-MAP decoder. In another embodiment, as an implementationalternative, if list decoding is employed in 4522, CRC Log-MAP decodingin 4526 could be skipped to reduce the processing complexity withoutsacrificing much the overall performance. In this case, and if CRC check4523 fails on all sequences, the LLRs and bit decisions of theinformation bits of the PSD PDU are obtained from the output LLRs fromblock 4522 and passed via lines 4532, 4534 and 4538 to blocks 4529 and4527. Thus, the CRC Log-MAP Decoder 4526 is optional, and is morebeneficial if only Log-MAP decoding is used in 4522, and provides lessgain when list Log-MAP decoding is employed in 4522. The output LLRs ofinformation and/or coded bits may be used for further iterativeprocessing shown in FIG. 46.

Processing of Audio

FIG. 42 shows one embodiment for decoding of audio packets. From astream of channel LLRs for an MPS PDU on line 4549, which is also shownon line 4496 in FIG. 40, audio frames each of which includes an audiopacket with a CRC field and any protocol control information (PCI) bitsare extracted in block 4541.

For each audio frame output from block 4541, list Log-MAP decoding isperformed in block 4542, similarly as described earlier for MPS PDUheader. The list Log-MAP decoder outputs decoder-output LLRs of theinformation bits in the audio frame and corresponding bit decisions, aswell as an ordered list of length M_value of hard decision sequences,from the most likely to the least likely, consisting of binary values 0and 1 on line 4551. In block 4543, PCI bits are then identified andextracted and their decoder-output LLRs are collected for additionalprocessing in block 4565. Block 4543 also outputs the bit decisions,their decoder-output LLRs, and the list of hard sequences of the audiopacket on line 4552 after puncturing PCI bits from the audio frame.Then, the CRC check is performed on the MAP bit decisions of the audiopacket in block 4544. If the audio packet passes CRC check (line 4553),its bit decisions are output as the final bit decisions by block 4563 toline 4564 for the information bits of the audio packet. Also, each LLRvalue is enhanced in block 4545 to a large value of magnitude preservingits polarity, e.g., 100 or −100 for logical bit 1 or 0 (or vice versa,depending on the binary bit mapping in the system). If the audio packetfails the CRC check in 4544 on the MAP decisions, the list of sequencesof the audio packet are then CRC-checked one at a time in block 4544 inthe order they are placed in the list until a path sequence in the listpasses the CRC check. Some path sequences in the list may be the sameand thus any duplicated path sequences may be removed before runningthis CRC checks in order to reduce the processing time required toperform this operation. When the first path sequence is found (line4553) that passes the CRC check, the path sequence is output as thefinal bit decisions in block 4563 of the information bits of the audiopacket and their LLR values are enhanced in block 4545 to a large valueof magnitude preserving its polarity, e.g., 100 or −100 for logical bit1 or 0. If no path sequence is found (line 4554) that passes the CRCcheck in block 4544, CRC Log-MAP decoding is optionally performed inblock 4547 on the decoder-output LLRs from block 4543 via line 4554,especially when list decoding is performed in 4542. Then, the LLRs forthe information bits of the audio packet are determined from the outputLLRs of the CRC Log-MAP decoder (either total or extrinsic LLRs) inblock 4547. Also, the final bit decisions for the information bits ofthe audio packet are made in block 4563 on the polarity of the outputLLRs of the CRC Log-MAP decoder. As an implementation alternative inanother embodiment, the CRC Log-MAP decoding in block 4547 could beskipped to reduce the processing complexity, without sacrificing muchthe overall performance. In this case, the LLRs and bit decisions of theinformation bits of the audio packet are obtained from the output LLRsfrom block 4543. When only Log-MAP decoding is used in block 4542, theperformance gain due to optional CRC Log-MAP decoding is larger than inthe case when block 4542 performs also list decoding. The output LLRsmay be used for further iterative processing shown in FIG. 46.

The decoder-output LLRs for PCI bits extracted from the audio frame areprocessed by the Log-MAP decoder in block 4565, by fully exploiting thestructure of PCI codewords. Block 4565 represents an additional Log-MAPdecoder designed to decode a short-length sequence such as PCI bitsobtained from a small set of code words. Once the PCI bits are processedby the Log-MAP decoder 4565, the output LLRs 4571 for the PCI bits arepassed to 4573 and their bit decisions 4572 are obtained from thecodeword decision on line 4570. The output LLRs may be used for furtheriterative processing shown in FIG. 46. Due to additional coding gain ofPCI codewords, PCI bits exhibit much better performance than audiopacket bits.

FIG. 47 shows the performance of the audio component of P1 logicalchannel for hybrid FM HD Radio systems. The performance using advanceddecoding implementing various embodiments is compared with performanceusing conventional methods. In this exemplary embodiment, it is assumedthat digital, OFDM signal power is boosted by 10 dB (i.e., −10 dBc)relative to the level allowed by the original HD Radio standard,according to an FCC Order in 2010. Slow urban fading channel, USLOW2, isconsidered. The inventive methods employ advanced initial CSI estimationand one additional iterative CSI estimation stage after first FECdecoding of P1 packet employing tail-biting Log-MAP decoding. InventiveR-matrix decoding is employed to facilitate CSI estimation. Afteriterative CSI estimation, tail-biting list Log-MAP decoding is employed,as described earlier. The list decoder uses M_value=32. The conventionalmethod includes a single stage CSI estimation using filter lengths overtime and frequency suitable for the range of mobile speeds and frequencyselectivity of the channel. The conventional method also usestail-biting Log-MAP decoder described earlier, which provides similar,if not essentially the same, performance as MATLAB tail-biting Viterbidecoder. LLR metric calculation is implemented in the form of a linearclipper, suitable for Laplacian noise, to account for impulsiveness ofhost FM interference that exhibits approximately Laplacian distribution.Such a metric yields better performance than a usual LLR metric forAWGN. It can be seen that at FER=10⁻⁵, the method according to certainaspects of the invention provides about 4 dB of gain.

FIG. 48 shows the performance of conventional receivers in USLOW2channel when reference subcarriers (R-matrix) data symbols used for CSIestimation are known and when they are reconstructed using R-matrixdecoding with conventional methods described earlier. It can be seenthat the receiver performance degrades by up to about 0.5 dB, in thisscenario, with CSI estimation based on demodulation of referencesubcarriers using the prior art method. In contrast, the advancedreceiver employing inventive aspects as described in relation to FIG. 47shows no performance difference, not shown in the figure, with CSIobtained using advanced R-matrix decoding and with perfect knowledge ofR-matrix.

FIG. 49 shows audio packet performance for the same channel model as inFIG. 47, but now also a first adjacent channel interference is presentat −20 dB relative to the host signal. This level of interference isillustrative of cases when first adjacent interference cancellationcould not work because it is relatively weak, or could correspond toresidual interference after applying first adjacent interferencecancellation. One may note that both inventive and conventionalapproaches degrade by several dB due to additional interference thatalso exhibits impulsive characteristics. However, the conventionalapproach degrades more, such that the inventive approach achieves about6 dB gain already at FER=10⁻³. Eventually, both receivers exhibit errorfloor performance, but the error floor for the advanced receiver basedon aspects of the invention is close to the target performance FER=10⁻⁵desired for commercial operation.

FIG. 50 shows audio packet performance in fast urban fading channel withan exemplary vehicle speed of 60 km per hour, UFAST60, and with a firstadjacent channel interference present at −20 dB relative to the hostsignal. The exemplary embodiments of the conventional and advancedreceivers employing inventive aspects are the same as those used togenerate FIG. 49, except that the conventional method uses MATLABtail-biting Viterbi decoder, which provides similar, if not essentiallythe same, performance as the tail-biting Log-MAP decoder describedearlier. The advanced receiver with the inventive approach achievesabout 7 dB gain in symbol energy to noise power ratio (Es/No) atFER=10⁻⁵ desired for commercial operation.

FIG. 51 shows audio packet performance in 3-ray fading channel, 3RAYSwith mobile device speed of 100 KPH, and with a first adjacent channelinterference present at −20 dB relative to the host signal. Theexemplary embodiments of the conventional and advanced receiversemploying inventive aspects are the same as those used to generate FIG.50. The advanced receiver with the inventive approach achieves about 7dB gain in Es/No at FER=10⁻⁵ desired for commercial operation.

FIG. 52 shows audio packet performance in UFAST60 described earlier. Theexemplary embodiments of the conventional and advanced receiversemploying inventive aspects are the same as those used to generate FIG.50. In this exemplary embodiment, however, it is assumed that digital,OFDM signal power is boosted by 6 dB relative to the level allowed bythe original HD Radio standard, according to an FCC Order in 2010, thusresulting in more FM interference seen by the digital OFDM signal thanin the 10 dB power boost case in the previous example/figures. Theadvanced receiver provides more gain than in the case of 10 dB OFDMpower boost in FIG. 50. Specifically, the gain is about 8.5 dB gain inEs/No at FER=2×10⁻³ and increasing as the conventional receiver exhibitsthe error floor. This demonstrates that the advanced receiver is morerobust with increasing interference compared to the conventionalreceiver.

FIG. 53 shows the performance of program service data (PSD) PDUcomponent of P1 logical channel for hybrid FM HD Radio systems. In thisexemplary embodiment, it is assumed that the length of PSD PDUs is 1000bytes. The exemplary embodiments of the conventional and advancedreceivers employing inventive aspects are the same as those used togenerate FIG. 49, and tested in the same channel model, USLOW2, with thesame first adjacent channel interference present at −20 dB relative tothe host signal. The advanced receiver with the inventive approachachieves about 7 dB gain in Es/No at FER=2×10⁻⁴.

FIG. 54 shows the performance of main program service (MPS) PDU headercomponent of P1 logical channel for hybrid FM HD Radio systems. In thisexemplary embodiment, it is assumed that the length of MPS PDU header is88 bytes long, including fixed header, variable header, and part of thePSD PDU. The exemplary embodiments of the conventional and advancedreceivers employing inventive aspects are the same as those used togenerate FIG. 47, and tested under the same channel model, USLOW2, withthe same level of a first adjacent channel interference at −20 dBrelative to the host signal. The advanced receiver with the inventiveapproach achieves about 3 dB gain in Es/No at FER=10⁻⁴. Similarly, FIG.55 shows the performance of main program service (MPS) PDU headercomponent of P1 logical channel for hybrid FM HD Radio systems with thesame embodiments for the conventional and advanced receivers, as well asthe channel and interference models, as those used in FIG. 49. Theadvanced receiver with the inventive approach achieves about 8.2 dB gainin Es/No at FER=2×10⁻⁴.

FIG. 43 shows one embodiment for decoding of PIDS PDUs. Using a streamof channel LLRs (after puncturing to a code rate 1/3 as in 4481 in FIG.40) for a predefined number of PIDS PDUs on line 4588, list Log-MAPdecoding in block 4581 is performed for each PIDS PDU. The list Log-MAPdecoder outputs bit decisions and their decoder-output LLRs of theinformation and/or coded bits in the PIDS PDU as well as a set (i.e.,list of length M_value) of hard decision path sequences of binary valueof 0 or 1 on line 4589. Then, CRC check is performed on the MAP bitdecisions of the PIDS PDU in block 4582. If the PIDS PDU passes CRCcheck (line 4590), MAP bit decisions are output as the final bitdecisions in block 4586 for the information bits of the PIDS PDU andeach LLR value is enhanced in block 4583 to a large value of magnitudepreserving its polarity, e.g., 100 or −100 for logical bit 1 or 0 (orvice versa, depending on the binary bit mapping in the system). If thePIDS PDU MAP sequence fails CRC check (line 4591), the path sequences ofthe PIDS PDU in the list are then CRC-checked one at a time in block4582 in the order they are placed in the list until a path sequence inthe list passes the CRC check. Some path sequences in the list may bethe same and thus any duplicated path sequences may be removed beforerunning this CRC check in order to reduce the processing time requiredto perform this operation. When the first path sequence is found (line4590) that passes the CRC check, the path sequence is output as thefinal bit decisions (in block 4586) of the information bits of the PIDSPDU and their LLR values are enhanced in block 4583 to a large value ofmagnitude preserving its polarity, e.g., 100 or −100 for logical bit 1or 0. If no path sequence is found 4591 that passes the CRC check inblock 4582, optional CRC Log-MAP decoding is performed in block 4585 onthe decoder-output LLRs on line 4591 (switched to line 4595). Then, theLLRs for the information bits of the PIDS PDU are determined from theoutput LLRs on line 4596 (to line 4597) of the CRC Log-MAP decoder.Also, the final bit decisions for the information bits of the PIDS PDUare made in block 4586 on the polarity of the output LLRs of the CRCLog-MAP decoder. As an implementation alternative, in anotherembodiment, the CRC Log-MAP in block 4585 could be skipped to reduce theprocessing complexity, without sacrificing much the overall performance,especially when list decoding is employed. In this case, the LLRs andbit decisions of the information bits of the PIDS PDU are obtained fromthe output LLRs from block 4581 passed to line 4597. When only Log-MAPdecoding is used in block 4581, the performance gain due to optional CRCLog-MAP decoding is larger than in the case when block 4581 performsalso list decoding. The output LLRs may be used for further iterativeprocessing shown in FIG. 46. The PIDS performance is better than theperformance of audio packets, due to a larger gain of list decodingcorresponding to shorter packet sizes used for PIDS PDUs.

FIG. 56 shows PIDS frame performance in 3-ray fading channel, 3RAYS, andwith a first adjacent channel interference present at −20 dB relative tothe host signal. The exemplary embodiments of the conventional andadvanced receivers employing inventive aspects are the same as thoseused to generate FIG. 51. It is also assumed that the length of PIDSframes is 80 bits long as described for the HD Radio systems. While theconventional receiver experiences relatively high FER, e.g., 10⁻², overa wide range of Es/No values, e.g., up to 12 dB, the advanced receiverwith the inventive approach (solid line with small dots) achieves about9 dB gain in Es/No at FER=10⁻² when advanced initial CSI estimation isperformed in combination of one additional iterative CSI estimationstage after first FEC decoding of P1 packet employing tail-bitingLog-MAP decoding. In addition, when the advanced receiver (dashed linewith circles) employs tail-biting list Log-MAP decoding after iterativeCSI estimation as described earlier, it further improves FER by about2.3 dB at FER=10⁻⁴ and achieves the target performance FER=10⁻⁵ (atEs/No<4 dB) desired for commercial operation.

Decoding of P3 Channel

FIG. 39c shows a stream of AAS PDUs, 4462-4466. Each of the AAS PDUscontains Flag 4467, data transport packet format (DTPF) 4468, datapacket 4469, and frame check sequence (FCS) 4470 (i.e., CRC). Each AASPDU forms an information block of length k bytes for an (n,k) RS code,e.g., k=223 bytes, and n=255 bytes. When RS encoding is performed, (n−k)RS parity bytes 4471 are appended to the AAS PDU to form a RS codewordblock 4472-4476. A group of consecutive RS codewords 4477 are thenbyte-interleaved with an interleaving depth of Rw (of typical value 4-64as in HD Radio AAS specification). In one exemplary embodiment, butwithout loss of generality, Rw=4 is considered. Not shown forsimplicity, the stream of RS blocks after byte interleaving is brokeninto a series of frames for inner convolutional code encoding atlayer 1. In the exemplary embodiment, without loss of generality, eachframe for convolutional encoding contains a sequence of bits of lengthcorresponding to two RS blocks (e.g., Rw/2=2).

FIG. 44 represents one embodiment regarding the advanced concatenateddecoder for HD Radio P3 channel carrying AAS data. The stream of channelLLRs corresponding to each convolutional code frame is de-punctured inblock 4601 by inserting zero values in the positions where the codedbits were punctured in the transmitter, to bring the coded stream tooriginal code rate. Each de-punctured frame on line 4616 is fed to thelist Log-MAP decoder 4602. The decoder generates a set of output LLRs,both information and coded bit LLRs, and a predefined number M_value ofmost likely hard-decision sequences (containing 0's and 1's) for eachframe. The list of hard decision sequences is ordered from the mostlikely sequence to the least likely sequence. M_value could be anyinteger larger than or equal to 2. Larger M_value results in betterperformance of list decoding, but the improvement diminishes beyond amoderate M_value, say 32. For P3 decoding in specific simulationexamples M_value=8 was employed. Tail-biting Log-MAP decoding isemployed in 4602, similarly as described in the context of MPS PDUHeader decoding description. Both optimal and suboptimal generation ofthe list of most likely sequences may be employed in differentembodiments. For computational efficiency, the size of M_value sequencesfor further processing is reduced in line 4618 as described below. Forthe AAS PDU structure shown in FIG. 39c , Rw/2 (i.e., 2) framescorrespond to Rw (i.e., 4) RS codewords. The output LLRs and thehard-decision sequences corresponding to Rw/2 frames are restructuredand de-interleaved in 4604 to generate a set of LLRs as well as a listof hard-decision sequences for each of the Rw RS codewords in line 4619.

For the specifics of the byte de-interleaving, refer to FIG. 45 whichdemonstrates the process for byte de-Interleaving of the hard-decisionsequences corresponding to Rw/2 frames from the list Log-MAP decoder inFIG. 44. After list Log-MAP decoding of each frame, the output containsone set of LLRs in line 4646 as well as a list of M_value hard-decisionsequences in line 4647. These sequences are ordered such that the firstone has the highest probability of being the correct transmitted framewhile the last one has the lowest probability of being correct. In orderto feed these outputs to the next processing block (de-interleaver), thefollowing steps are performed:

-   -   1. For a tail-biting convolutional code of constraint length k,        the last (k−1) bits of Head should be the same as the last (k−1)        bits of the packet for a hard-decision sequence to be correct.        Therefore, the sequences that do not satisfy this condition are        removed from the list while keeping the order of the remaining        sequences. This results in M₁≦M sequences.    -   2. Among the remaining M₁ sequences, some sequences may be the        same as the others in the list. Therefore, only the unique        sequences are kept that are a collection of the lowest-index        sequence from each group of the same sequences. This results in        a list of M₂≦M₁ sequences with their order in the list        unchanged. Steps 1 and 2 are performed in block 4641.    -   3. Remove Head and Tail parts from the LLRs and sequences in        block 4642.

As mentioned before, each set of Rw/2 frames corresponds to Rw RScodewords, in this exemplary embodiment. After removing Head and Tailparts, the soft LLRs from all the frames are sent to a de-interleaverwith depth Rw in block 4645. Therefore, at the output, LLRs of each setof Rw/2 frames are converted to Rw set of LLRs each corresponding to oneRS codeword, line 4658. Eight LLRs corresponding one byte symbol aremoved together to match the byte interleaving in the transmitter. As itcan be seen, de-interleaving of soft LLRs is straight forward. As forthe hard-decision sequences, the task becomes more complicated. Considera set of Rw/2 frames each with a set of hard-decision sequences obtainedusing the three step process above. The numbers of the hard-decisionsequences for these frames may not be the same and are denoted as{M ₂ ⁽¹⁾ ,M ₂ ⁽²⁾ , . . . ,M ₂ ^((Rw/2))}

The number of combinations will be M_(total)=M₂ ⁽¹⁾×M₂ ⁽²⁾× . . . ×M₂^((Rw/2)). Each combination can be fed to the de-interleaver of depth Rwin block 4645 to form a set of Rw sequences each corresponding to one RScodeword. In the end, for each of the Rw RS codewords corresponding tothe set of Rw/2 frames, M_(total) hard-decision sequences are obtained,line 4659. However, one issue remains to be addressed. As mentionedbefore, M₂ sequences of each frame are ordered from the highest probableone to the lowest probable one. When combining sequences from differentframes, different combinations have different probabilities of beingcorrect. For example, the combination of the first sequences from allRw/2 frames has the highest probability of being correct. Therefore,according to certain aspects of the invention, the combinations are fedto the de-interleaver in order of their probabilities of being correct.This way, the M_(total) hard-decision sequences of each RS codeword arealso ordered from the highest probable one to the lowest probable one,in an approximate fashion as described in an example below.

As an example, for Rw=4, each set of 2 frames contains 4 RS codewords.Assuming the frames have M₂ ⁽¹⁾, M₂ ⁽²⁾ sequences, the combinationsshould be of the following order:

$\begin{matrix}{( {1,1} ),( {2,1} )} \\{( {1,1} ),( {2,2} )} \\{( {1,2} ),( {2,1} )} \\{( {1,1} ),( {2,3} )} \\{( {1,3} ),( {2,1} )} \\{( {1,2} ),( {2,2} )} \\\ldots\end{matrix}$where in (x,y) above, x denotes the frame number and y denotes thesequence number. This can be performed as follows:

-   -   1. Generate all combinations (x₁, y₁), (x₂, y₂) such that x₁,        x₂=1, 2, y₁=1:M₂ ⁽¹⁾ and y₂=1; M₂ ⁽²⁾ in block 4643.    -   2. Sort the combinations in step 1 such that y₁+y₂ has an        ascending order in block 4644.

After de-interleaving, for each RS codeword, the M_(total) sequences online 4619 in FIG. 44 are checked in block 4605 to see if any of them isa valid RS codeword, meaning the RS (255,223) code is used for errordetection. If any of M_(total) sequences is a valid RS codeword 4620, itis further tested to see if it passes the CRC check in block 4606. TheCRC check provides an additional level of error detection capabilitybeyond that provided by the RS (255,223) code. If CRC passes 4621, thesequence is converted to enhanced LLRs on line 4632 and is used as theoutput where information bits can be extracted from it. Similar to thediscussion above pertaining to MPS PDU decoding, enhanced LLRs are usedfor iterative decoding by giving more weight to bit LLRs of the segmentsthat are considered correct, compared to LLRs from other segments, whichwill slightly improve iterative decoding or CSI estimation. In case noneof the M_(total) sequences is a valid RS codeword 4622, hard decision RSdecoding using Berlekamp Massey (BM) algorithm is performed on at leastL_(BM)≦M_(total) of the M_(total) sequences in block 4607. Similarly, ifnone of valid RS codewords passes the CRC check in block 4606, the setof M_(total) sequences is passed to block 4607 for BM decoding asdescribed above. In other embodiments, instead of BM algorithm otheralgorithms known in the art may be employed in 4607. Since thehard-decision sequences for each RS codeword are ordered based on theirprobability of being correct, in FIG. 44 block 4607 BM decoding isperformed on the first L_(BM) sequences starting from the highestprobable one. This increases the chances of the BM decoder to decodefaster and results in more efficient implementation.

If BM decoding was successful on any of these L_(BM) sequences 4625, theresulting highest ranked RS codeword is fed to CRC check block 4609 andif CRC passed 4626, it is converted to enhanced LLRs on line 4632 and isused as the output. However, if CRC check did not pass 4628 or BMdecoding was not successful on any of the L_(BM) sequences 4627, theLLRs corresponding to the RS codeword are fed to a SISO, ‘soft’, RSdecoder in block 4611, which has been explained in detail in previousembodiments of the invention. If soft RS decoding resulted in a valid RScodeword 4630, it will further be tested in the CRC check block 4613 andif CRC passed 4631, it will be converted to enhanced LLRs and used asthe output on line 4632. If CRC did not pass 4634, the input LLRs to thesoft RS decoder are used as the final output on line 4637 or they areoptionally passed through a CRC Log-MAP decoder in block 4614 and itsoutput LLRs are used as the final output of the whole decoder. If softRS decoder did not result in a valid RS codeword 4633, it generates aset of updated LLRs based on soft RS decoding employing BP decoding, asdescribed in the corresponding embodiments of the invention (seediscussion pertaining to choosing the best LLR where it is stated thatthe average of LLRs from all matrices is the best choice in terms of biterror rate and should be selected as the final LLR). These LLRs are usedas the final output on line 4637 or they can be optionally passed online 4636 through the CRC Log-MAP decoder in block 4614 and its outputcan be used as the final output LLRs. The output LLRs may be used forfurther iterative processing shown in FIG. 46 by constructing from themextrinsic information as known in the art.

It should be mentioned that list Log-MAP decoding is primarily used forreduction of decoding complexity. The reason is that in most caseseither one of the sequences is a valid RS codeword and passes the CRCcheck or one of the sequences may be decoded with the simple BM decoderand pass the CRC check. In both cases, the more complex soft RS decodingis skipped and the overall complexity is reduced. In another embodiment,Log-MAP decoding may be used instead of list Log-MAP decoding in block4602. Log-MAP only generates a set of soft LLRs that go to line 4618 forbyte de-interleaving. The output at line 4619 goes directly to block4611 for soft RS decoding and everything else in between is skipped. Ithas been shown that using list Log-MAP decoding with BM before soft RSdecoding yields better performance compared to a combination of Log-MAPand soft RS decoding.

FIG. 57 shows the performance of P3 logical channel for hybrid FM HDRadio systems. The performance using advanced decoding implementingvarious embodiments is compared with performance using conventionalmethods. In this exemplary embodiment it is assumed that digital, OFDMsignal power is boosted by 10 dB (i.e., −10 dBc) relative to the levelallowed by the original HD Radio standard, according to an FCC Order in2010. Fast urban fading channel, UFAST60, is considered, with both hostFM and 1^(st) adjacent interference. The inventive method employsadvanced initial CSI estimation and one additional iterative CSIestimation stage after first FEC decoding of the P3 packet. In addition,inventive R-matrix decoding and tail-biting list Log-MAP decoding aswell as inventive soft RS decoding are employed, as described earlier.The list decoder uses M_value=8. The soft RS decoder is the combinationof the original proposed decoder and the alternative embodiment withdisagreement positions as described earlier. Both the original proposeddecoder and its alternative using disagreement positions use N_mat=6matrices. In addition to the common first (n−k)−L (n=255×8=2040,k=223×8=1784) degree 1 columns, each matrix has a different set of Lcolumns with degree 1 as discussed earlier. (L=18 for the originaldecoder and L=15 for the alternative decoder with disagreementpositions). For each matrix, 7 rounds of sorting and matrix adaptationis performed. During each round, 3 iterations of simple greedy BPalgorithm with α₁=0.2, β₁=0.3475, g₁=0.61 (for the original decoder) andα₂=0.18, β₂=0.43, g₂=0.62 (for the alternative decoder with disagreementpositions) are performed except for the last round where 9 iterationsare performed. At the end of iterations for each matrix that has notconverged to a codeword as well as the input LLRs and the average LLRsof all 6 matrices, BM error and erasure decoding is used. For each setof LLRs, all symbols with probability of correct smaller than 0.4 areerased, such that the number of erasures does not exceed (255−223=32).As mentioned earlier, during the decoding process, hard decision RSdecoding using Berlekamp Massey (BM) algorithm may be performed onL_(BM)=3 sequences in block 4607. The conventional method includes asingle stage CSI estimation using filter lengths over time and frequencysuitable for the range of mobile speeds and frequency selectivity of thechannel. The conventional method also uses tail-biting Log-MAP decoderdescribed earlier, which provides somewhat better performance thanMATLAB tail-biting Viterbi decoder. It can be seen that at FER=10⁻⁴, theadvanced receiver (corresponding to the line with ‘plus’ symbols),implemented as described earlier, achieves a gain of 1.5 dB relative tothe conventional receiver (corresponding to the line with ‘triangle’symbols). In another exemplary embodiment, multiple iterations betweenouter RS decoder and inner convolutional Log-MAP (or list Log-MAP)decoder could be used to further improve BER and FER performance, asdiscussed in other embodiments.

Iterative Decoding in HD Radio Systems

FIG. 46 shows one embodiment for iterative decoding of the MPS, SIS, andAAS PDUs. The signal on line 4688 represents the outputs from blocks4067, 4068, and 4069 in FIG. 24b . These streams of channel LLRs arede-multiplexed into three logical channel streams for further decodingof MPS PDUs in block 4672 (also block 4071 in FIG. 24b ), SIS PDUs inblock 4676 (also block 4072 in FIG. 24b ), and AAS PDUs in block 4677(also block 4073 in FIG. 24b ), respectively. The three Informationdecoders output bit decisions for their respective PDU as well as LLRson line 4692 for P1 MPS PDU, on line 4693 for SIS PDU on PIDS, and online 4694 for AAS PDU on P3. The LLRs for each PDU may include enhancedLLRs for segments that have converged to correct codewords that pass CRCcheck as well as LLRs for non-converged segments (and also extrinsicinformation if iterations need to continue). A decision is made (block4679) as to whether or not the iterative decoding should continue.Unless all PDUs are correctly decoded or a predefined number ofiterations have reached, the three streams of enhanced LLRs andextrinsic information are supplied to SISO Decoders as a prioriinformation for their respective output bits, i.e., the MPS PDU streamon line 4692 to block 4681, the SIS PDU stream to block 4682, and theAAS PDU stream to block 4683, respectively. The updated coded bit LLRsat the output of SISO decoders 4681-4683 may help to improve the CSIestimation in another round of CSI estimation. All of the output codedbit LLRs from the SISO decoders on lines 4695, 4696, and 4697 areproperly interleaved and multiplexed in block 4684 for the same signalformat as the outputs from block 4040 in FIG. 24a . Interleaved andmultiplexed coded bit LLRs are mapped to a desired soft or hard symbolsto facilitate CSI estimation in block 4685. Then, blocks 4685, 4686, and4687, respectively, perform CSI estimation, symbol-to-bit de-mapping toget updated channel LLRs, and de-interleaving as previously described inreference to blocks 4064, 4065, and 4066 in FIG. 24b . Then, the outputsignal on line 4699 from block 4687 containing updated and more reliablechannel LLRs for all information source streams 1, 2, and 3 is suppliedto block 4671 for the next iteration of the processing in the subsequentblocks.

In summary, as discussed in FIG. 24b , first one or more iterationsbetween CSI estimation and SISO decoding may be performed for the radioframe containing PDUs of multiple information sources. This helps toimprove the performance of CSI estimation and consequently more reliablesoft information at the output is given to information decoders 1, 2 and3 for different PDUs. “Good” bits from successfully decoded PDUspropagate to other parts of transfer frames and overall radio framecomprising multiple information streams to improve their performance,via SISO decoders 4681-4683 and Advanced CSI Estimation 4685. Then a fewglobal iterations are performed between information decoders 1, 2 and 3,4672, 4676 and 4677, SISO decoders 4681-4683 and Advanced CSI Estimation4685. These iterations improve the reliability of LLRs at the output ofinformation decoders which result in more reliable decoded informationsequences and consequently improved performance of the whole system.

Although not explicitly shown in FIG. 46, the output LLRs from blocks4672, 4676, and 4677 may go through additional processing for thespecific PDU format, as applicable, such as byte-interleaving for thesignal on line 4694, as the decoding of AAS PDUs in block 4677 involvedbyte de-interleaving. But these additional processing omitted in FIG. 46do not alter certain aspects of the invention that are focused oniterative processing of the enhanced output LLRs from the InformationDecoders through SISO Decoders and Advanced CSI Decoder, which were alsoplaced in the forward path for the non-iterative decoding process shownin blocks 4064-4069 in FIG. 24b . An illustration of the performanceimprovement with iterative decoding is shown in FIG. 57. For theconsidered channel scenario described earlier for the example in FIG.57, the iterative advanced receiver using one more decoding iteration(corresponding to the line with ‘circle’ symbols) provides an additionalgain of about 0.25 dB, compared to using one more decoding iteration,compared to the advance receiver with only a single decoding stage. Itshould also be noticed that additional iterations are required veryinfrequently in the region of FER of interest.

While there have been shown and described various novel features of theinvention as applied to particular embodiments thereof, it will beunderstood that various omissions, substitutions and changes in the formand details of the systems and methods described and illustrated may bemade by those skilled in the art without departing from the spirit ofthe invention. Those skilled in the art will recognize, based on theabove disclosure and an understanding therefrom, that the particularhardware and devices that are part of FM HD and AM HD Radio systems, andthe general functionality provided by and incorporated therein, may varyin different embodiments of the invention. Accordingly, the particularsystem components shown in FIG. 1-FIG. 57 are for illustrative purposesto facilitate a full and complete understanding and appreciation of thevarious aspects and functionality of particular embodiments of theinvention as realized in systems and methods thereof. Those skilled inthe art will appreciate that the invention can be practiced in otherthan the described embodiments, which are presented for purposes ofillustration and not limitation, and the present invention is limitedonly by the claims which follow.

What is claimed is:
 1. A system for generating check-to-variablemessages during an iteration in decoding of codes represented by aparity check matrix, the system comprising: a de-mapper configured toreceive a modulation signal comprising symbols, and convert themodulation signal into coded bit log-likelihood ratios; and at least onedecoder coupled to the de-mapper, wherein the at least one decoder isconfigured to receive the coded bit log-likelihood ratios from thede-mapper and iteratively perform message passing decoding for a set ofvariable nodes identified in the check parity matrix so as to generate adecoded signal comprising bits, and wherein for at least one variablenode in the set of variable nodes during the iteration, the at least onedecoder is further configured to: a. calculate a check-to-variablemessage M_(CV)(i,j) from check node i to variable node j, b. identifytwo smallest absolute values, Min₁ and Min₂, in a set ofvariable-to-check messages M_(VC)(i,k), where k≠j, excluding the messagefrom variable j to check node i, M_(VC) (i,j), c. calculate a scalingfactor ${\alpha = {1 - {\beta\frac{{Min}_{1}}{{Min}_{2}}}}},$ where β isa non-negative number such that 0≦β≦1, d. scale the check-to-variablemessage M_(CV)(i,j) as M_(CV)(i,j)=α·M_(CV)(i,j), e. update check node iand variable node j with said scaled check-to-variable messageM_(CV)(i,j), and f. output the generated decoded signal.
 2. The systemof claim 1 further comprising at least one de-interleaver coupled to theat least one decoder.
 3. The system of claim 1 wherein the at least onedecoder is further configured to calculate the scaling factor α for aplurality of variable nodes connected to check node i, and to: a.identify three smallest absolute values, L₁, L₂ and L₃ in a set ofvariable-to-check messages {|M_(VC)(i,:)|}, where L₁ and L₂ correspondto variable nodes jmin₁ and jmin₂; and b. calculate a first scalingfactor${{\alpha( {i,{j\;\min_{1}}} )} = {1 - {c\frac{L_{2}}{L_{3}}}}},$a second scaling factor${\alpha( {i,{j\;\min_{2}}} )} = {1 - {c\frac{L_{1}}{L_{3}}}}$and a third scaling factor${\alpha( {i,j}\; )} = {1 - {c\frac{L_{1}}{L_{3}}}}$ wherej≠{jmin₁,jmin₂} excluding the messages from variables jmin₁ and jmin₂ tocheck node i, where c is a non-negative number such that 0≦c≦1.
 4. Thesystem of claim 1 wherein the scaling factor a suppresses more largermessages and less smaller messages.
 5. The system of claim 1 wherein thecheck-to-variable message M_(CV)(i,j) from check node i to variable nodej is calculated using a min-sum algorithm.
 6. The system of claim 1wherein the check-to-variable message M_(CV)(i,j) from check node i tovariable node j is calculated using a sum-product algorithm.
 7. Thesystem of claim 1 wherein calculating the check-to-variable message,identifying the two smallest absolute values, calculating the scalingfactor, scaling the check-to-variable message, updating the check nodeand variable node and outputting the generated decoded signal arerepeated until each check node i is updated.
 8. The system of claim 1wherein the at least one decoder comprises a soft-input soft-output(SISO) message passing decoder, in which the check-to-variable messagesare passed based on best graph belief propagation.
 9. The system ofclaim 8, wherein the decoding based on best graph belief propagationuses multiple parity check matrices.
 10. The system of claim 1, whereinthe at least one decoder comprises a soft-input soft-output (SISO)message passing decoder, in which the check-to-variable messages aregenerated during decoding of Reed-Solomon codes represented by a paritycheck matrix of size (n −k) ×n.
 11. The system of claim 10, wherein allnon-binary elements of the parity check matrix are replaced by m ×mbinary matrices resulting in a (n −k)m ×nm binary parity check matrix.12. A system for modifying variable-to-check messages during aniteration in decoding of codes represented by a parity check matrix, thesystem comprising: a de-mapper configured to receive a modulation signalcomprising symbols, and convert the modulation signal into coded bitlog-likelihood ratios; and at least one decoder coupled to thede-mapper, wherein the at least one decoder is configured to receive thecoded bit log-likelihood ratios from the de-mapper and iterativelyperform message passing decoding for a set of variable nodes identifiedin the check parity matrix so as to generate a decoded signal comprisingbits, and wherein, for at least one variable node in the set of variablenodes during the iteration, the at least one decoder is furtherconfigured to: a. calculate variable-to-check messagesM_(VC)(i,j)^((n-1)) and M_(VC)(i,j)^((n)) from variable node j to checknode i, respectively, in iterations (n−1) and (n), where n >2, b.compare the positive or negative signs of variable-to-check messagesM_(VC)(i, j)^((n-1)) and M_(VC)(i,j)^((n)) calculated in step a, c. ifsaid signs in step b are different, generate modified variable-to-checkmessage {acute over (M)}_(VC)(i,j)^((n)) according to {acute over(M)}_(VC)(i,j)^((n))=gM_(VC)(i,j)^((n))+(1−g)M_(VC)(i,j)^((n-1)), where0≦g≦1, d. update check node i and variable node j with said modifiedvariable-to-check message {acute over (M)}_(VC)(i, j)^((n)), and e.output the generated decoded signal.
 13. The system of claim 12 whereinthe variable-to-check messages M_(VC)(i,j)^((n-1)) and M_(VC)(i,j)^((n))from variable node j to check node i are calculated using a min-sumalgorithm.
 14. The system of claim 12 wherein the variable-to-checkmessages M_(VC)(i j)^((n-1)) and M_(VC)(i,j)^((n)) from variable node jto check node i are calculated using a sum-product algorithm.
 15. Thesystem of claim 12, wherein the at least one decoder comprises asoft-input soft-output (SISO) message passing decoder, in which thevariable-to-check messages are modified during decoding of Reed-Solomoncodes.
 16. The system of claim 12 wherein calculating thevariable-to-check messages, comparing the positive and negative signs ofthe variable-to-check messages, generating modifiedvariable-to-check-messages, updating the check node and the variablenode and outputting the generated decoded signal are repeated until eachcheck node i is updated.
 17. The system of claim 12 wherein the modifiedvariable-to-check messages minimize error propagation by converging to acodeword.
 18. The system of claim 12, wherein the weight g is set suchthat 0.5≦g≦1 in order to give preference to the newer messageM_(VC)(i,j)^((n)).
 19. The system of claim 12, wherein the at least onedecoder comprises a soft-input soft-output (SISO) message passingdecoder, in which the variable-to-check messages are passed based onbest graph belief propagation.
 20. The system of claim 19, wherein thedecoding based on best graph belief propagation uses multiple paritycheck matrices.